Apple has announced some support for external GPUs in masOc High Sierra, and the Radeon 580X is one of the few card that seem to be supported. Although as a single GPU in a MacPro we probably wouldn't see the splash screen at startup. The card is hard to come by though. I am still on Sierra, and I might wait until the end of the year to upgrade os + GPU. We'll see.
The card has seen a lot of usage, and has been working flawlessly, so I'm not complaining. I read on the internet that the 7950 glitches happened in a single monitor configuration. Might be why I have been immune on my station since I am using two monitors. My previous post about this problem was back in 2014 in a MacPro 3,1: https://humanuser.blogspot.com/2014/02/radeon-7950-mac-edition-on-macpro-31.html
