Compared to other exhausts it's probably the most quiet exhaust, some people like that though, look into the Kinetix V+ plenum its meant to give +15hp i think it is.
Maybe there's no problem at all if you haven't noticed that the car has gotten slower, it could just be the power that it has. My V35 is the fastest car i've owned by a long shot, i hit 60 in town so fast i have to be careful.