The stock tranny is strong. It is rated at much more TQ than you will get from any normal mods, especially for running on the street. I haven't even heard of any drag cars getting exploding trannys or diffs.
What does happen, is the entire drivetrain flexes and so smaller components, like bushings, toe links, etc can go bad. This can lead to some funky stuff if it happens at speed and your rear wheel starts to turn.
About the most out really is reinforced toe links. They are the ones that can cause harm (loss of control), should they fail. the bushings are simply a nuisance.
As for gains, I'd say adding 100hp and 100TQ is about it without some crazy stuff. A few have gone higher, but only by mixing and matching mods.