There's been a few gaming posts, but I haven't read about anyone who has tried WoW full screen. I heard about someone using a 24" and someone running it on a 21, but not at the native resolution.
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)
actually i have the i7 imac and I run in complete Ultra settings... shadows and all in full screen mode and i get 45-60 fps in 25 mans... In Onyxia 25man I stood in the middle of the whelpss being AOE'd during Phase 2 and with all the aoe going on I saw it drop to 25fps....
the i7 runs so incredibly smooth any lag i've noticed in game has been completely server side