I've had HD7950-R9280X in my 1,1 and now in 4,1. 1,1 PSU is fine so don't worry about that (it's 980W, same as 4,1/5,1 just with different connections to motherboard).
AFAIK, compatibility with 1,1/2,1 is not some hack by the users but Sapphire's gift to us
so original Mac edition also works with 32bit EFI. If i remember correctly, MacVidCards mentioned that there are traces of 32bit drivers in official AMD driver pack for 10.7.5-10.8.3, but are not finished. So if you want to run HD7950 in 1,1/2,1 you have to pass 10.8.3 and use Apple drivers (but i see that you're running 10.11.6 so no problem there).
I've upgraded to HD7950 from HD6850 and the jump in performance was huge (for only 50euros more).
P.S. I really don't see why would you buy Sapphire Mac Edition when regular HD7950 work (flashed or not) and the price difference is huge.