Many people have broken 915 boxes with engines producing hp and torque in the 300 range (including Brandon, two times
). The 915s that hold up are a fluke and/or the owners are dainty with the throttle (no sudden bursts of acceleration), which is hardly the point of building a blown 911 engine.
The high hp, n/a engines have nowhere near the torque of a boosted engine, so are easier on the 915.
Matt Monson posted side by side pics of 915 v 930 gears, the 915's are far more narrow/skinny - easy to see why they aren't up to the task.