I have been following your posts and what you learned from using the 1080 Ti and decided to get the RX 580. It works great with FCPX and Resolve.
I think the RX 580 is a great card to put in a Mac Pro, if you're OK with the kext mods. It's around half the performance of a 1080 Ti, but less than half the price. Also, it's a card where your Mac can utilise all of the card's potential across the board with no reservations.
The 1080 Ti is a powerful racehorse that you pay a lot for to use. In the scenarios where it can stretch its legs (my DaVinci Resolve tests and
certain Maxwell Render use cases) you get all of your money's worth. But then there are many cases where you don't see its full potential. There are also some instabilities—some people might be fine with these, but for me they were a show stopper.
Even not considering the show stopping side of things, since there are some issues where you don't see full performance, that makes the price/performance aspect less attractive.
If you need/want equal performance to a 1080 Ti, you could run two RX 580's for less money. Apps need to be multi GPU aware, which FCPX, Motion and Resolve are. I don't use PPro (and only little AE), so I'm not speaking to that.
It seems that Nvidia GPUs work great for gamers while video application users are better off with AMD GPUs, perhaps with the exception of Premiere (that I don't use).
This isn't quite in line with what I've said. I DO think it's a strong gaming card if you get full acceleration. But I didn't with F1 2016 (RX480 was stronger) and the 1080 Ti hung the computer. In X-Plane I'm CPU limited. with my two 3.46GHz Xeons.
DaVinci Resolve is a really attractive choice for editing/color/sound right now. The full Studio version is only $299, plus there is an almost complete free version you can simply download. The 1080 Ti is a perfect card for Resolve on the Mac. In fact, if editing and color was all I did, I'd easily keep the card and be happy! Strong recommendation!
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Barefeats has recently published a big comparison of the various Pascal cards, and multi-card setups in the cMP. Makes for some interesting reading.
Barefeats is a great site*. If you follow it, you'll have nice factual information almost as quickly as if you read Macrumors Mac Pro forum.... =)
My reservations to his latests tests with so much compute power is that he should move up from 1NR node to the 6NR node test. His 60 fps score with dual 1080 Ti is limited by things other than the cards. With tougher tests (available in the benchmark) you'd see the same linear increase (double performance) maintained with the two 1080 Ti's all the way.
*= No irony. It's a great site!