Coheed went through 3 or 4 p11 gearstacks in his se-l at around 450whp before finally going to the 6 speed and now hasnt had to worry about it and hes now at just over 500whp give or take.
The type of gear oil you use has a lot to do with how long they last too. As ive said a million times and ill say it again. Valvoline 85w-140 Conventional or Durablend FTMFW. Anything over 300whp on our transmissions I would recommend using it. Better, smoother shifts, much more cusion on the gear stacks, and overall quieter and much better at taking the abuse. Hell if I could get a b13 trans to hold 450 to 550whp for 9k miles of torture both street and track and still not break it with a twin disk clutch after switching, Id say that shows something being I broke 3 stacks in 3 months prior to switching to that oil from a 75w90 oil.