Hi,
I only just got the 2x converter so not a lot of experience with it just yet and haven't tried it with AI Servo yet...
On the 5D MkII the AF is a little slow in low light with some hunting. On the 7D, however the AF seems fine. Not superfast like normal when it focuses in an instant but still quick enough for events and haven't experienced any hunting as of yet.
I haven't tried this combo's AF performance under tough conditions, such as fast moving animals, kids or indoor sports but I wouldn't feel confident with the 5D MkII but would expect a decent hit rate on the 7D.
Hope that's of some help,
Damon
7D/5D MkII, 2xTC MkIII and 70-200mm f2.8L IS II