Im running 30% in mine with stock shims. I use 30% in all my motors for the most part. 30% seems to give u the lil extra punch on the bottom end. Also, try a short mugen header(MUGJX14002) with the 2045 pipe. Ive tried it on the c6 and it really opens the motor up more and it doesnt seems to lose hardly any bottom but it screams alot harder on the top end.