guys at the moment i have a full decat 3'' to 2.5'' downpiepe tsl, decat centre 2.5'' tsl and firestorm back box again TSL

can someone tell me what bhp will this be good for?

due to restriction on the gas flow will i need a full 3'' system if i wanted to take my car up from 330bhp nearer (car is a p1)

also what about high flow sports cats how good are these and what power can be achieved from these as i would like less of a headache come mot time (and of course the environment thing lol)

steve