I have a MacPro3,1 and a PowerColor R9 270 GPU that I would like to use together. Unfortunately, this card does not work out of the box unlike my previous Radeon 6870; I never flashed it, but I never had a problem waiting for the login screen for the display to come to life.
According to hackintosh documentation, this GPU should be usable on the Mac by spoofing the device ID using Clover. However, from what I've seen so far it looks like Clover is a replacement for the Mac's EFI, or at least a modification to it which I'm uneasy about.
Is it advisable to follow this path and use Clover on a real MacPro3,1? If this was a Hackintosh, I wouldn't think much about it, but I do want to keep this machine somewhat pure to avoid any compatibility issues. If I have to sell this GPU and pick up something else, so be it.. but I'd like to think that it isn't that far off to get the R9 270 to work on this machine since it is so close to other devices that are supported officially by Apple.
(BTW: the card does work in the machine as it functions under Windows 7 / BootCamp)