I installed a Helix downpipe this past weekend from Gruppe-S. The turbo sound is very noticable and it mates perfectly to the stock catback. I'm alittle disapointed with the gains. I don't really feel it. Maybe i need to reset my ecu. I will get an uppipe and maybe then i'll feel a bit more difference, a bit more oomph. All i know is that without two of those cats, my car reeks like stinky exhaust fumes. It's pretty bad.