the S54 isn't really a high torque motor. I've taken apart an E46 front diff and it's really not that weak either - the E30 ix's front diff is much weaker, and it'll take 300-400hp. And isn't the TC gear driven? there seem to be a lot of xi front CV shaft problems though.
the biggest weak point is the xi AWD system being based completely on the brakes adjusting the torque vector. It probably works fine but your brakes are going to be working extra hard.
the ZF trans from a 6-speed xi should very easily be able to handle the S54's ~260ft/lbs of torque. Hell, you can build an M20 that makes that much torque. it's not like it's a twin turbo V8 or something. I don't think I'd want to do it with the M3 trans, there are drivetrain alignment issues to consider - the Getrag needs to be the exact same length with the adapter or the mounts for the TC and driveshafts will no longer fit. It makes more sense to use the ZF 6 speed. The ratios are almost identical anyway.
Usually BMW drivetrain parts can handle at least 50% more torque than they see in service, if not more. the M54B30 doesn't really make much less torque than an S54 either - about 20ft/lbs.
Last edited by hassmaschine; 12-31-2012 at 11:10 AM.