I cant speak for the OS differences, but on my Windows 7 rig, which contains a Radeon 4850 GPU and a Core i7 CPU, I average 35-55 FPS at 2304x1440 in Trial of the Crusader 25, running in windowed mode.
I would say 25-35 for the iMac, due to the slightly higher resolution, OS differences, and clock speed differences. When it comes to WoW, when theres a lot of people on the screen, I've noticed its more CPU-bound on your FPS than GPU-bound at that point.
(I have a 4850X2, but the second core goes unused in WoW)