Quite a bit of good info in the threads on the AWD section, we've discussed the XI drivetrain quite a bit.
IMO, there really isn't enough information to suggest a weak point yet since many failures are with stock units. Basically when you break it, then take it in for a tear-down and they can help us with stronger versions of what has failed. I didn't want to be the first 335xi to find the limit, but I've got an addiction to speed and can't help myself
I've got a TNT this Friday to see what the car will run with nitrous + FBO + Meth, after that I'll do a dyno to see what the AWHP is.