mouse24 :
Call me crazy but couldn't you just flash the R7s firmware with a different vendors firmware? If it worked going from a slightly different card (6950 to 6970 I think it was, don't quote me) then surely this wouldn't be an issue?
Also I have edited my posts above saying my advice was wrong, because it was, and I shall go reread up on ultra fast boot. Sorry about the misinformation OP.
All vendor VGA firmware is derived from a reference firmware image provided by AMD/NVidia. The differences are in how it controls cooling, power regulation, temperature monitoring, clock derivation, etc... Little things like that. Some vendors have published updated firmware images that include the UEFI GOP support, but many have not. Some have even published updated firmware images that claim to include UEFI GOP support but have done so in a completely inoperative fashion (I'm looking at you Gigabyte). You may even be able to find a modded firmware image on some forums, but I would advise against using these as they are published by modders, not engineers who have access to the appropriate technical documents. Overwriting the firmware image with one from another vendor may work, or it may cause it to malfunction (wonky fan speed for example), or it may brick it completely.
I would advise against updating the VGA ROM with anything but a newer image from the appropriate vendor.