I was thinking that if ATI Firepro and Radeon are used in real Macs they should be fully-supported in Mavericks and Yosemite. Then why there is no mention about such GPUs in 2015 August Buyer's Guide?
I'm currently working on a (soon to be released) Radeon compatibility guide which will answer your questions.
FWIW, my Sapphire Vapor-X 280X works OOB.
My experience wasn't quite OOB. I had to do this extra setup to get the system to boot from a non-USB drive.
http://www.tonymacx86.com/general-h...so no initial config was necessary. :thumbup: