I gotta disagree, going from a stock exhaust to a 2.5 inch no cat, no resiantor, one see through muffler exhaust, I gained downlow power and uptop power. the car actually moves at sub 1500 rpm. you can put ur foot on the go pedal to make it step out sideways at 3000~ rpm.
Oh and yeah i also have 2" extractors.. speaking to an exhaust guy he said all the "back pressure" you need is usually gotten at the extractors/exhaust manifold/headers.