At what point do people generally go to a 31 spline diff/axles? How much can the 28 spline setup take on slicks? 400lb/ft torque? 500? 300? Is there any drawback to upgrading these parts - like rotating mass, more rolling drag, whatever?
As far as durability, I have seen stock 5.0's break axles, ring gears, and diffs from high reving launches on slicks. As such, your answer is if the car is driven hard, then 300tq can snap/twist stock axles. As far as drawbacks to upgrading, there are really none, other than cost. The gain is durability, while rotating mass/friction are the same. If you upgrade to a 9'' ford, then you gain friction because of the larger mass and ring/pinion size. In any case, it seems miniscule.