More Air (and fuel) = More HP. Engines are air pumps...
Loud is bad? Hmm, he's tired of the ricer fart cans running around (can't say I blame him), but just because a car is loud doesn't mean its fast, nor does it mean its slow. On our cars, Catbacks are mainly for sound, you MIGHT gain 5whp with one, is it worth the $600 +/- for the 5hp? Its not IMO.