Its not going to improve much to be honest........pos at a push 1 to 2 bhp. The GTI standard airbox has been tried and tested a lot and is not that restrictive at all. I have smoothed the inside of mine and added a panel filter like most and also changed the intake pipe for better cold air flow/pick up the main box (this part is a crap design to me) . I have also drilled a few holes in the base of the box (small section) with another cold air feed added to these and she drives great I had a BMC and it was ok but did not think much of it to be honest and sold it on.