There's a few other variables here that are impossible to test unless you did a before and after on a dyno, but small "problems" with timing/fuelling might be assisted by extractors (or more importantly, the better flowing exhaust system that comes with them) which is where some of the "under the curve" gains that MightyMouse is referring to might occur.
I still vote that there will not be any miraculous gains. It might well feel a "little bit better" but for 90% of people I would say in a blind test (and without a change in exhaust noise) you wouldn't pick it.
PS cars with extractors on them don't have to sound any way in particular either. In a mate of Greg's postman's car, we ran a G13BB with mandrel bent extractors

into a 2" system with a high flow cat, one 14" hot dog, a triple flow muffler and a 12" hot dog.
It is soooooooo quiet, you can barely hear it at idle and you only get even a hint of note on big long rips at full throttle to high revs.
At idle, the injectors ticking are louder than the exhaust.
It's really nice to spot a car when you can talk to the driver whilst he is driving.
Steve.
[quote="greg"] some say he is a man without happy dreams, or that he sees silver linings on clouds and wonders why they are not platinum... all we know, is he's called the stevie.[/quote]