I am the proud new owner of a 2001 Impreza RS . I got the car about 5 weeks ago and have 850 mles on the clock. The car is amazing. I have already been bitten by the aftermarket tuning bug and plan on installing the exhaust system from JC. I am not sure how the different piping diameters affect the performance of the car. Could someone let me know. Thanks. See you on the road.