As we all know, the Mac Pro community (especially 2019) is pretty small. I can't say I'm super surprised to find your post here. To complete your story, from my perspective—I was the one who picked those up, sorry:
I have gone to MacSales (OWC) on a semi-regular basis, as I missed a sale they had more than a year ago where they sold W6800X Duos for something like $2400 or whatever, to see if I could upgrade my 2019 Mac Pro with two Pro Vega II (singles) to W6800X Duo over the long term and at a good price.
When I get a hunch, I check the Swiss local sales platforms like Tutti, Ricardo, and occasionally eBay. When I went to eBay a few days ago, you can imagine my surprise when
I found two W6800X Duos going for around $3000 for BOTH. Then again, the idea is: to offer for a pretty low price and let interested parties bid on them until their true market value is reached (whatever that is). Of course... if could also just be a scam.
There was already one bid placed at the starting price of 2800 CHF. I set a reminder for when the action was going to end to see what it looked like at that point. When I went there, I still only saw one (your) offer. I placed a bid and saw I was registered for 2810 CHF. The clock just kept ticking down and nothing happened... until it reached 0 and it said: 'congratulations, you are the winner'. I was kind of scratching my head at that point.
Anyway, the seller was oddly enough pretty local, and yesterday we met in person, talked a bit about 3D rendering woes, shook hands and parted ways... with me carrying a
substantial load of two large and heavy MPX modules.
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I also own a MacBook Pro 16 M3 Max, so I know exactly what both of these platforms can do. Most people should just go with Apple Silicon and never look back. It is actually turning out to be a really great general computing platform. Even in 3D workflows, when you actually WORK on your scene, viewport performance is great on M3 Max.
At the same time, the Mac Pro with two W6800X Duos is faster than my M3 Max (with hardware raytracing), but not by that much. The M3 Max is in turn faster than an M2 Ultra, so the hierarchy in Blender is:
two W6800X Duos --> M3 Max 40 --> M2 Ultra
This is just looking at pure performance and isn't taking price, size, or power into account. I'll reserve final judgment until they work perfectly in Blender. As it is now, two Duos work perfectly in 3.6.8 but there are some issues that are being addressed in version 4+.
In DaVinci Resolve it looks like this for a few selected Macs: