The other high-power Xis I know of (jpsimon, presidio335xi, etc) and I haven't had any issues with the AWD system. Unless you are launching hard you are unlikely to hit any AWD-specific issues. If you are launching hard then it's just the transfer case question.
Haven't had to replace a TC, so can't comment on costs.
07 335xi Monaco Blue | FFTEC/Vishnu Single Turbo | Procede Rev 3 | Spec Stage 2+ Clutch | Spec Aluminum Flywheel | ETS FMIC | HKS Legamax Exhaust | Vishnu PWM Meth