This Cat-Back system is a simple bolt-on.  100% stainless steel inside and out.  Twin round tips fill out the rear valence perfectly with no rubbing.  Not too loud, not too quiet.   Stock system weighed in at 45.5 LBS, the Borla weighs 33.5 LBS.  With our in-house Dynapack Chassis Dyno, we tested many different exhaust sytems currently on the market for the Cooper S.  The Borla came out on top!  Dyno results showed a peak gain of 6-7 HP power at 6250 RPM, but power increases from 3700 RPM to redline.