I went through a B13 transmission with about 300whp and it just rolled the teeth off of second gear, wasn't from a hard shift or anything. Not to say that it wasn't fatigued from hard shifts but when it went it was just normal acceleration, not even hard acceleration. This was with a sprung clutch.
My B14 transmission the same thing, except third gear just stripped out and I was at about 325whp. But it may have also been from hard driving and it just decided to go. This was with an unsprung clutch
I just went through a B15 transmission because of hard shifting (with unsprung clutch) and I'm guessing I was around 350-375whp when it happened. This transmission should have been fine if you just ease into each gear and then step on it...
My guess is that with a sprung clutch, easy shifts, and good lube
you should be okay to around 275-300whp but don't be super surprised if the gear stacks separate under throttle and your gears let go in a B13/B14 tranny at those levels.