The only true way of telling is to get the car on a dyno. that would give you an accurate reading of bhp and torque,
Remember the cars were meant to be 280bhp from standard,, so if you have the usual good xhaust system and a good filter this should help alot the the breathing and air flow, it may improve on the figures abit,,
My e4 has a full 3" turbo back xhaust blitz filter plus i have a gizzmo ms-ibc boost contoller fitted with different settings, on high setting i am seeing upto 1.5bar.. which sum say isnt safe for a 4 but its been hitting that ever since i have had it and it seems a ok,,, I had mine on the rollors just after xmas and it was seeing 312bhp and similar torque figures,,