Stepping up to a wider or stickier tire will be rough on the differential. I can't personally recall hearing of anyone breaking the stock axles on street tires but I'm sure there is some yahoo out there that has managed to do it.
Odds are that with your mods you will be aok - I'd bet you will wear out the differential before the axle breaks.
Step up to a sticker tire and all bets are off... ask me how I know. :-D
Buddy of mine broke a stock axle in his 96 cobra...but it was from hitting a curb when he hit a ice patch on a road just kicked the car over and right into the curb.
Wider and or stickier tires will cause faster wear on bearings, axle, and diff.
I ran stock axles in my car for 4 yrs no probs. I run a 275/40/17 rear tire (falken azenis rt615-pretty grippy tire). The only reason I dont have them now is cuz I sideswiped a curb and bent one. I run superior 28 spline ones now. Oh it on the motor in the sig. Makes about 350 to the wheels and I run a 3.55 rear end.