Hi David,
Breaking differential input shafts is more down to the behaviour of the driver rather than a serious misgiving in terms of shaft strength. Whilst it is true that they are the weak link in the differential, you'll really only run the risk of breaking one if you drive in such a manner where traction is lost. The point at which traction is regained is where damage can be done.
My Rover runs a 4.6 litre engine which delivers some 50% more torque compared to the 3.5, and I have not had any problems (so far, typing very quietly).
Ron.