The best performing and quietest exhaust systems are typically custom built. You can get 2.5" piping (stainless, aluminized, etc.) and match it up with a resonator and muffler. The larger the resonator, the less the noise. I built an exhaust for my old car with pipe from www.kteller.com and a Magnaflow 18" resonator and a Magnaflow 5x8 muffler. Everything was 2.5" and very quiet. The car sounded stock at idle, had a nice low non-droning tone while cruising, and screamed at WOT.