Alright, so I got the card and obviously had the same issue with the jpg quicklook as you guys.
Decided to try 18,3 on an external Mojave install but this only worked using Lilu, Whatevergreen AND iGPU, talk about OOTB experience
I guess this would probably work for 14,2 too but by enabling the iGPU my boot time crawled from less then 10s to 1m (didn't time it though)
Then I tried 6,1 without iGPU and it worked, fast boot again and no issues with jpg. It's probably placebo but MacOS even feels snappier and Power Management has more states. I've seen 8, 40, 41, 44 so far unlike 8 and 44 with iMac 14,2.
Then I removed Lilu and Whatevergreen and lo and behold, everything still works. So no kexts nor iGPU required using MacPro 6,1.
I then moved the MacPro 6,1 config from the USB to my main install while following
P1LGRIM's iDiot's guide to iMessage because I didn't want to mess up my already working iCloud & Messages and again all was well. Let's just hope things continue to work out in the long run using this SMBIOS.
As for the RX580. The issue I thought I had with my GTX970 seems to be related to the monitor profile created with Colormunki Display that shows a blue color cast when trying to edit in Photoshop from Lightroom. If I knew this before I don't think I would have bought the RX580 because everything else feels the same and I have to again lower my Photoshop GPU settings to basic to avoid the issue (basic avoids the GPU for color profiling in Photoshop)
What surprised me was the performance in Windows which seems to be the same or even less then the GTX970 in some games.
Also I was going nuts over the stalling mouse issue, looked it up and lo and behold it's a known issue that has a temporary fix by using mouse trails in settings, hmmm.
So I can't say it was a smooth transition to AMD and those with the GTX970, hopefully they will release drivers soon because this card is still top notch. I'm keeping it for another week to see if the RX580 (and Mac Pro 6,1) stays stable during work.
Thanks to all who share their experience and expertise with us, without you guys all this wouldn't even exist!