This is my point exactly, i've seen drops in power on other cars due to heat interference, just because a car sounds throaty does not mean more power.
A lot of these so called sports exhausts do absolutely nothing until the engines screaming for mercy at it's rev limit, but at least they sound good.
Has anyone ever done a rolling road test before and after fitting things like this to prove they are getting more power, rather than it just sounding better therefore it must be better?