how much difference does a performance exhaust really make to the performance of a car, i have a totally standard exhaust at the mo and am considering getting one the turbos from the group buy with a 3" downpipe tapering off, question is if i got a sports cat and say a 2.5" or 2.75" exhaust what difference would it really make as oppose to just having the standard 2.25" exhaust, i am just looking at performance per £ spent as know a sports cat with a stainless steel exhaust system back would be about £500, not interested in noisy exhausts like the fact mine is quiet, cheers guys