The stock Cooper S performs fairly well for a stock exhaust, but it's heavy. The least expensive way to improve performance is to remove the resonator (so-called "one-ball" mod.) If you're looking for a more elegant solution, consider the Alta Stainless Exhaust.
The ALTA Performance exhaust system combines a show quality exhaust with a high horse power tuner system. Featuring 2.5" diameter tubing for a perfect match to the high revving, supercharged power plant in the Cooper S. The enlarged dual exhaust tips will retain an aggressive factory look.
Each NTK Oxygen Sensor is: Extensively Tested During Manufacture to Guarantee Quality and Reliability Supplied With a Factory-Fitted OEM Connector for Easy Installation Precision manufactured with a sensing element incorporating NTK's premier technical ceramics
BORLA Type-S / Type-S II Cat-Back & Rear Section exhaust systems feature patented straight-through and multi-core technology to unleash hidden horsepower. Increased exhaust velocity adds power, driving excitement, fuel economy, and a distinctive more aggressive sounding “S-Type” sound - respected by motoring enthusiasts everywhere.