As I said, my rear drive gives what seems like high torque to me (I think bike's at about the power you mention). I've only taken my bike off-road a bit but it seems like the force on the rear hub and the bottom bracket are things that simply have to be designed for. The bottom bracket experiences both frame flex and the rider pounding the pedals, so it's subject to punishment and the rear hub is cushioned by the tires, spokes and any rear suspension present.