For what it's worth, a friend of mine had a 94 HCI 150 shot GT that he launched off the limiter on slicks and sprayed it out of the hole for about a whole summer with a stock rear end except it had 3.73s in it, then he built a new rear end and we put his stock one in my old pullied '04 Cobra which I put about 25k more miles on, some with DRs, some without, and some racing also, and it never broke. Perhaps we had a freak, but the 3.73s were setup wrong and whined on decel a little , so I installed some new ones a couple thousand miles before I sold the car, but the axle splines and diff still looked and worked good. I had a rebuild kit for the diff. expecting it would need it.
Now on the other hand plenty of people have busted up stock axles/diffs with 300rwhp and DRs. Most failures on the stock rears that I've seen are the splined part of the axle breaking. I think it's a good idea to upgrade to 31 spline stuff. $275 for axles, under $100 for a used 31 spline T-lok, $80 or so for a T-lok rebuild kit, maybe $50 for misc. stuff. When upgraded axles/diff. become necessary I think the torque boxes should also be reinforced/welded along with the axle tubes welded. Just my 2 cents.