I run my 2015 Cooper S Automatic in SCCA solo D-street (against the WRX's and now the FRS/BRZ twins without Toyota tuning kit). I'm looking to buy a catback system if I can realize even a little better low RPM throttle response (torque figures below 2000 rpm for instance). Peak HP gains would be nice too, naturally. Some events I run are at high altitude and the automatic could use a little help when leaving the line as well as maybe downshifting coming out of corners. The downshifting isn't too big a deal, I can fix that completely by simply anticipating more and getting back on the throttle sooner. - but, Wow, overall, the automatic works really well in solo! I had people coming up to me at the last event at high altitude in Sierra Vista saying they were amazed at how fast it upshifts. I had 3d quickest time in about a 15 car street tire indexed PAX class. Would have had first if I had thought to try power-braking leaving the line during the official runs. Tried it later during time-only fun runs and the car was definitely faster. No one else in the Street tire Indexed class went that much faster during time-only's So, I'd like to find the best cat back that works without an ECU tune to remain legal in D-street. I'm leaning towards a dual mode exhaust like the JCW tuning kit hardware (but w/out the ECU tune), the Armytrix valvetronic, the Milliteck dual mode, or maybe a single mode like the NM or quicksilver.