well to make 500bhp the engine needs to consume more air,with standard cams the valves won't be open long enough for a start.

so the engine may only make 400bhp with 1.5bar on a T4 as it would with over 2bar on the same turbo,the extra boost/air flow is just backed up in the inlet because it can't be consumed, i reckon you can only force so much before you start to bottle neck

that's my theory anyway shame this isn't in GD for more discussion though