1. yes you can play wow on it no problem on very decent settings.
2. it doesnt really slow down the macbook at all. I haven't formatted in years and it still chugs along fine.
3.Cata should work fine. i'm playing now at 60fps on just under max settings with shadows down and we have most of the cata patches installed now since the game comes out in 2 weeks.
4.my macbook pro does get hot. i control the fans with smc fan control and a laptop cooling pad. I run at 83*c
5.why install windows when it runs fine in mac os x? Unless you want to play windows only based games you should have no need for windows.

In short. yes it runs great and runs in macos x fine as its designed as such from our great developers at blizzard.