Finally, here are my results:
Core i7 960 @ stock 3.20 Ghz
6GB 1066Mhz DDR3
Gigabyte X58A-UD3R rev 2.0
Sapphire (reference) 5870 1GB
64-bit Graphics Enabler = yes
Tried Mac Pro 3,1 4,1 and 5,1 identifiers. 3,1 is the fastest, but I think it just forgoes power management altogether. The difference is so small that I'm currently running on 5,1.
My Sapphire 5870 is reference design, but the fan idles quite loudly. Would there be any benefit to adding the device ID to the apple power management kext? Does someone have a link explaining how to do this easily (i.e. how to find my device ID).
Cinebench is running around 26.5 fps, and Starcraft 2 runs at 1080p on extreme settings without dipping below the 60fps max I set for the game.
The system is very stable, but I do get a KP about 20% of the time when booting, although a quick restart remedies that easily.