Alot depends on mechanical sympathy. i mullered 3rd gear in my old classic at 280ftlb, but was on the mountain pass on the isle of mann hahaha, its torque that kills box's rather than bhp.
I ran my type r 5 speed at 330bhp/380ftlb but was only for about 4k miles and i knew it was being pushed so no hard launches. As said 330-350ftlb is typically as far as you'd would push a 5 speed, only exception being a hawk 5 speed, for some reason they seem to be taking alot more punishment than others. I ran 400/420 on one for about 14k miles with no problems and know a few that have been run over 450ftlb without problems.