I just took a 915 out of a car making 370 HP and 350 ft/lbs - at the wheels - in favour of a G50. It was in there for quite a long time (6 years); they don't necessarily grenade if the 915 is treated with some basic respect and common sense (don't just yomp on the loud pedal at 2000 RPM, no drag starts etc).
But that 915/72 got built up (LSD, oil pump cover, spray bar kit, WEVO bearing retainer) to replace the '77 915 that a stock(ish) 3.3 chewed up and spat out (before my ownership, so I actually have no idea what kind of shape the original box was in before the turbo blued the 5th gearset and turned the cases to soap around the bearings).
There's an old thread on this on the 911 forum
http://forums.pelicanparts.com/porsche-911-technical-forum/250779-how-beef-up-915-thread-parts-costs.html
I'm pretty sure that when Hayden says "350 ft/lbs" he means at the flywheel - like the factory would. It's only the turbo guys that quote at the wheels.