My mate used ot run a Silent HiPower system on his (now) 500bhp S14a and the GT SPec he has now is more free flow. The bung saps a little power for sure, but who cares!?!? Its only for track days!
I fitted the HKS Silent Hi Power and its awesome mate...thanks to Nicely
who advised me on this system...i would definately recommend it as it gives good increase in power and relatively quiet even without the cat...