You will get the most power gains with an aftermarket downpipe. The N55 sounds so great that we can't get it loud enough, in our opinion at least.
Catted downpipes offer more turbo noise and greater volume but may not give you and increase of crackles and burbles to the level you are looking for. Catless downpipes will let
all the aggression through as long as you can tolerate a CEL or a tune.
Alternatively, we have a
Muffler Bypass Exhaust which is a straight pipe rear section to take things to the next level if a mean sounding exhaust is a priority. If you have any questions, we'd be happy to chat with you.