My 05 diesel (6 speed ) was remaped and the exhaust modified to 615Nm 3 years ago and I personally haven't experienced any problems, and torque is by far more of a problem to the the drive line than Kw ( HP ) A point I made to one of our members who asked me about performance increase to X5s was I would think it far less of a strain on the transmission or any other part of the drive line by remaping or supercharging than it would be towing at somewhere near capacity for extended periods, My E53 has a towing capacity of 2.5 tonnes (metric) think about the exponential forces of pulling that up an incline or the increased load at speed due to wind resistance of the towed vehicle. In AUS the same trans has been used for many years in our GT Falcon (Ford) which developes around 428hp/433fp (590nm) so obviously Ford decided the 6 speed was well capable, and still has a tow capacity above this. Cheers.