I have a 2002 gt tdi pd150 i have had it superchipped and believe i have around 190bhp can anyone tell me how much extra i would gain if i was to put exuast system without cat on and do i need a new downpipe or is it ok to cut cat out then put system on from there back,what about k&n filter any good or not should i change this or just leave that standard.also i hear about dump valves,power boost valves and air flow meters can anyone explain how these work on my car thats if they do im not sure.