You are correct, but the MBP GFX card is noticeably slower than the iMac's and has been shown to be under-clocked to control heat.
The higher thermal envelope in the desktop does indeed help, and allows the faster CPU/GPU speeds far earlier than the notebook. And as Fonzijr1964, the lack of a battery is also a major plus.
Still comes down to deciding on how much you want to spend on duplication of equipment, and how much you want to spend to get a specific GPU in the machine -- and whether that money is worth it.
Looking at barefeats will help, if they have the benchmarks for the game you are interested in. But $2200 for a machine just to play WOW is pricey.