i haven't seen any reports of the tranny ever failing under high horsepower. There are some v8 projects in the works that use the e153 box so I bet there will be some broken trannys in the future. The CV joints seem to fail every once in a while though. you can get upgraded ones from ATS.
the synchros are weak on the 91-92 trannies i beilieve only a single or double on 1st, whereas the 93 box had beefier triple cone syncrhos on 1-3rd iirc. not sure about upgrading just the internals or if you need the whole 93+box.