Failing that, I'm not clear how you would get a card-specific EFI BIOS for boot screen support. If that assumption is correct, then it would lead me to believe that adding an EFI BIOS that works on a similar card (R9 280X or 7950 which are both close but not quite) should work since I'm assuming that the existing BIOS would be similar enough to hook in. To some degree, I've recently come away with the notion that you don't get rid of the existing BIOS, but rather add an EFI partition to it that the Mac would recognize.
I've seen numerous guides on the internet about flashing the BIOS on the video cards and also ones that speak about adding the EFI BIOS, but in the haphazard forum approach, it isn't clear how this all works. The card is supported on the Mac once it boots, but I would like to get boot screen back since my old 8800GT has been baked in the oven a few too many times by now :-) When combining the 6900xt and the 6800xt, they are considerably faster than the 2x Vega ii pro. From my tests below from a Mac rumors benchmark, it seems like a single 6900XT is almost as fast as 2x Pro Vega ii in blur, and actually faster in TNR vs 2x Vega ii. Also seeing lots of Nvidia 8800GT for sale. The ATI HD5770 is a supported card upgrade for my model. I've got a PC video card that I'd like to get boot screen support for on my Mac Pro. This is why I love the 7.1 Mac Pro - the 6900XT is an amazing upgrade for video. Learning Mountain Lion is possible on 2006 Mac Pro 1,1, but not with my stock Nvidia 7300GT.
