Okay, so I finally put my money where my mouth was and did this. Herewith my experiences, with amusing asides to note the places that worked and didn't.
First and foremost: when doing this, it is VERY important to keep your application in mind, and to understand and work with certain limitations. There WILL be some. (See my post above.)
I decided to go this way for one supremely stupid reason (well, "stupid" aside from the fact that it was costing me earnings): Second Life. As I think I mentioned elsewhere, SL is a very badly optimized virtual environment, and it taxes the hell out of even the best gaming GPUs. On-chip graphics are often brought to their knees, and running SL a lot can significantly shorten the life of a computer unless it is very well cooled.
To say that modern Macs are not very well cooled is like saying that the Unabomber had socialization issues.
My 2014 Mini worked great for audio and music work, simple video editing, streaming, and the like. It was nice to have USB3 and Thunderbolt 2 for high-speed stuff, and it was hooked to a T2 hub that let me use a second monitor etc. However, the CPU was cooled by one tiny, anemic, very poorly placed fan... and even if I remembered to use Macs Fan Control (great app, pop $15 for the Pro version just to be nice, even though the free one works well for most folks) to kick the fan up to full blast before launching SL, I was still getting thermal issues and kernel panics that brought me down in the middle of online paying gigs. The money lost was one thing, but the blow to my reputation (and my radio station's) was basically unacceptable. (Also, have you ever tried to relax and enjoy doing something while waiting for shit to blow up? NOT fun.)
So, as much as I enjoyed the Mini, it had to be replaced by something that could run SL without barfing. (And which could bring back some of the advantages of a big open box with places to, you know, PUT stuff.)
Some research got me the following information: Mac Pros are all upgradable to some extent or another, but some are better than others. The 3,1 is the worst (that's the one I used to have) and the 5,1 is the best. With the arrival of Catalina, prices pushed down hard by Mojave have gone even lower; it really is NOT hard to find a reputable reseller on Ebay moving these models for quite decently low prices. Differentiators include internal components, RAM amount, and most especially CPU. The Mac Pro had a whole bunch of different Xeons in it over its life span; faster, slower, different architecture (Westmere vs. Nehalem etc). People will pay a premium (up to a point) for more and faster cores.
However, in my case, this was a non-issue. I am unaware of any DAW or other music software that will run on Mojave that makes truly efficient use of multicore architecture. (That is not a rhetorical statement, I literally don't know. If someone does, post below, please?) Even if there was, my applications wouldn't usually be core-intensive, so I felt could buy a cheaper machine with more RAM and a less desirable (for most folks) CPU.
I found my Mac in a hurry; a very reputable reseller was selling one for under $700. Quad Core Xeon running at 2.8 GHz, 32 GB RAM, Radeon HD 5770 installed, already with Mojave on it (thereby proving it could be done!). It shipped out and arrived, and I got my first unpleasant surprise: like many Mac Pros of the era, this one took a hit in the past, and the metal legs that it rests on were badly deformed. This doesn't affect performance at all, but makes the computer ungainly and ugly. I propped it up with old mouse pads, made it comfortable in its new cubby (remember the old computer hutches with vertical cubbies for big desktops?), and named it "Clonk."
To be able to run Mojave, the Mac's GPU needs to have Metal graphics capability, and to do THAT on a Mac of that vintage, you need a very specific generation of GPU. Too old and they won't do Metal; too new and they won't work at all in that Mac. The 5,1 usually shipped with an AMD Radeon HD 5770; this was what came in mine. It was a perfectly fine graphics card for 2011, with 1 GB of dedicated GDDR3 RAM, but by today's standards it's pretty pokey. I could use one for SL but it wouldn't be a lot better experience than the GPU in the Mini... well, aside from the crashing.
Fortunately, a quick search on the Internet reveals several pages that refer to an announcement made by Apple about which Metal-capable graphics cards would work in a 5,1 and allow the installation of Mojave. The HD 5770 is one, but there are better ones out there... assuming you can find them.
Here's the trick: with very few exceptions (i.e. none that I've found but I'm not a psychic), the "recommended but not proven to work" list, which talks about generic classes of GPUs rather than specific tested models, is pretty solid across the board, and some of the cards on that list are still being made, albeit in modern forms. Hunt around on sites like newegg.com and you'll find cards that fit the bill.
I spent under $200 on an XFX-brand card based on the Radeon RX 580 architecture, which is still put in really inexpensive gaming PCs built by dads who want a starter machine for the kids. Most of the other options on the list are either gone or fabulously expensive, but the RX 560/570/580 are commonplace and relatively cheap.
With the GPU taken care of, I then went down to Micro Center. For those of you who don't know what Micro Center is, it's basically a huge big box store with so much computer tech in it that it's hard to find something they DON'T have, including a lot of obsolete tech for folks trying to eke the last bit of life out of old PCs. (My friends and I nickname it "Nerdstrom's.") And this is where I ran into my second gotcha: I already had my GPU from my local Best Buy, as it was price competitive and equal to if not better than anything in stock during the pandemic at Micro Center... but that machine needed a lot more stuff... well, not needed, but you know. And the lot more stuff cost a lot more. You need to build this into your budget -- many of the components in that Mac will be old and serviceable, but you will want to replace them, and I quickly found out that adding all the gubbins I wanted doubled the price of the Mac!
Mojave installed on a 9-year-old Seagate hard drive? Yeah, NO. Replace that sucker with a 1 TB Solid State Drive. I like to use SSDs for many applications, including recording audio and storing sample and virtual instrument libraries; I purchased a second one for that. For that one, I succumbed to temptation and went to a 2 TB SSD, which are now becoming quite affordable. (Sorta.) For both, I got the Crucial MX500 series; the BX500 is marginally cheaper but has a very bad reliability/lifespan rep because it's using "lower costs no matter what it does to performance" tech. Samsung EVO 860s would also have been a great choice, but at the moment they weren't on sale so the Crucials were significantly cheaper, and I mean SIGNIFICANTLY (like 33%).
I also needed adapter cradles to plug the 2.5" SSDs into the Mac's 3.5" drive bays; they're about $15 each for the good quality metal ones. I figured I'd keep the old 1 TB HD and use it for scratch space, and to do internal Time Machine backups, I grabbed a 4 TB Toshiba NAS-spec hard drive. That would fill the four main bays, and if I ever needed one more SATA device, there was the empty bay under the DVD drive (which I didn't bother replacing).
I panicked for a moment over whether or not the Mac would be able to power the new card; some headscratching revealed that the two mini-connectors on the motherboard could be connected to a Y-cable that would in turn feed the GPU's power input, no extra cost required. I also needed a DisplayPort-to-HDMI converter box because the card I bought had only one HDMI port; this was cheaper and potentially more useful than buying a dedicated adapter cable, assuming MicroCenter had one, which they were all out of, sorry sir, it's the pandemic.
My purchases in hand and the sticker shock of the butcher's bill over with, I went home, opened up Clonk, removed a bizarre little PCI card that the previous owner apparently used for capturing HDMI gaming video and that the reseller left in as it wasn't hurting anything, and loaded up all the bays with the various drives. Closed it up, fired it up, sent up a prayer to the Gods of Computing... and by God, it came up the first time!
However, my relief was shortlived. The hard part was easy; now the easy part would turn out to be hard.
I was ready to populate the new Mac's drives, and I had created a bootable clone of my Mac Mini's SSD on an external USB3 HD. Backing this up to one of the new SSDs would serve as a starting point for my new machine's OS drive. The external drive also had a partition with the audio content for the second SSD, as well as bootable backups of my other computers (hey, back up one, back them all up while you're at it, right?). Plug it in, boot from it, run SuperDuper!, reassign the startup drive, done.
Well, it would have been done, if not for one teensy problem. The reseller neglected to provide me with one important bit of data: the admin password for the Mac. No changing the startup disk, bucko. You have to boot from the internal 1 TB, and you can't run cloning software on it because you can't install it because guess what, you don't have the admin password! Contacting the seller through Ebay could take days, and he might not even remember it or have it written down.
OK, now what? I tried to boot from the external during startup, but I discovered one little quirk that some GPUs have when you try to run them this way without doing some special flashing of the firmware: you lose the boot screen where you can do things like force the use of another boot drive. NOW what?
This is where my Achilles' Heel kicked in. I got CLEVER. Instead of stepping back and trying to think of what would be simplest and fastest, I started haring off down tangents that cost me time and sweat with no success. Maybe I could create a bootable clone on a drive that would only have one boot volume instead of multiples? Let's try that. Wait, the internal drive STILL wants to boot. With the monitors dark, we can't see what's wrong....
Hours went by; my family started avoiding me as my "I'm FINE!" got angrier and angrier (research a novelty record called "Daddy's Curses").
Finally I was sitting on the floor in the studio glaring at the Mac, so tired I was ready to go to bed and try again in the morning... and in my sleep-fogged state, I just snapped. My "FUCK THIS!" further terrified my wife and daughter, but I ignored their cries of panic as I got down to some pretty dirty business... which in the long run actually probably saved time over any elegant method.
Peripheral note that is critical to the story:
Mac users who don't work with Windows tend to forget that one of the more interesting low-level features of macOS has been missing from Windows since about the era of XP (and which I don't believe has returned in 10?): Unlike Windows, which makes you go through a relicensing process any time you significantly change the hardware configuration in a computer (or try to move it to a new computer entirely), macOS honestly doesn't give a shit what computer you put it in.
At all. Ever.
As long as the hardware is physically capable of booting with a particular OS, it will, no questions asked, no driver problems, nada. Works EVERY time. (Unless you're building a Hackintosh, in which case you get no sympathy from me.)
That was how I had planned to get going. My old Mac Mini's OS was cloned onto that external backup drive, but nothing I could do in a neat and pretty way could get it onto the Pro and supersede the existing OS on that shitty old hard drive. So I chucked "neat and pretty" and got MEDIEVAL.
Damn Mac won't let me get in and do anything? FINE. We'll go back to one that WILL! (I yanked the monitor, keyboard, and trackpad cables out of the back of the Pro and bunged them back into the Mini.)
Damn Mac won't let me set up my drives nicely? FINE. Let's see how NOT-NICE works! (I plugged in a special USB drive caddy that lets a user plug in one or two bare drives, either 3.5" or 2.5" SATA, and slapped in the two blank SSDs. Filled one from the OS of the Mini -- which should work fine in the Pro, remember? -- in about half an hour, then did the same for my audio content drive in about 45 minutes. Gotta love USB3 transfer speeds on SSDs, baby!)
Damn Mac wants to throw a hissy fit and ignore my nice new SSDs when I put it all back together? FINE. See if you can do that NOW! (I ripped out the existing old HD -- remember that at the time, this was the ONLY drive that would verifiably boot the Pro? -- slammed it into the caddy, and ERASED IT.)
Okay, you ugly cranky piece of mechanical SHIT. I've scrubbed your fucking brain. What are you going to do about it NOW? (I put all the freshly populated drives, yes, even the 1 TB I had erased, back into the Mac, closed it up, reconnected everything, and hit the Power button.)
The Mac tried to boot. Its OS was gone, missing admin password and all. Its thought process was something like this:
"Okay, booting u-- uh-- hey... hey, where's my BRAIN? My brain is GONE! Oh calamity and WOE! Mercy, O Gods Of Silicon! MERCY! Poor pitiful me, all alone in the dark with nothing at all rattling around in my poor empty head! However shall I -- waitwaitwait. What? What's that over there? A new brain? Oh, okay. Cool. We're good."
Booted up in seconds. Everything worked. Done. And aside from reauthorizing some plug-ins, it's been tootling along nicely ever since. And boyoboy does Second Life look good with 8 GB of GDDR5, and boyoboy does Clonk run cool with five, count'em, FIVE gigantor fans, including two on the GPU itself. Audio apps work on the first try, everything copacetic, done.
Now, keep in mind that I am not trying to do ANY of the next-level stuff Stephen opened this thread with. I don't care about overclocking or getting Thunderbolt to work or anything like that; I just wanted a big solid Mac with plenty of expansion space and plenty of graphics grunt and plenty of places to plug stuff in, and I got it. See my previous posts for reasons why I consider all that stuff to be just showing off, especially for music work.
Have I sacrificed anything? Well, yes, a bit. For one thing, the PCIe speed in the Pro is only 1 GHz. That's Thunderbolt 1. Not only does that mean that Thunderbolt peripherals are useless, but it also means that USB3 support is also impossible. USB2 is fine for my work, but if I ever needed super-low latency multitracking, I would have to go hunting for a PCI card for that. Easy enough to find, and I have the slots for it. Hell, I could even use an old FireWire interface again now!
I have decided to keep my Mini. USB3 transfer speeds have proven useful, I might need Thunderbolt 2 for something someday, and hey, it takes up practically no room. I have dedicated an old Bluetooth keyboard and trackpad to it, left its old Thunderbolt 2 dock and external DVD drive on it... all I need to do now is get an HDMI switch, so I can use one of my existing monitors on it and give up dual screen work for a short while.
One last thing I haven't researched yet is that it's unclear whether the Pro will get maximum throughput on the SSDs. If they're designed to a SATA standard that the Mac doesn't understand, they'll fall back to a slower default one. In other words, if the Pro only knows about SATA I and SATA II, but the SSDs are set up for SATA III, then the Pro will not know what that means and will play it safe, defaulting back to running them at SATA I speeds. I have yet to see if this makes a huge difference to my particular workflow.
But aside from the idiocy of getting the OS up and running, this was an amazingly painless process, and I am loving the results. Your mileage may vary, but if you keep your expectations reasonable (and don't go freaking overboard on internals like I did!), this is an economical setup to keep working well into the brave new world of Catalina, Apple processor Macs, and beyond.
Phew. Time for dinner. Stay safe, all!