I wouldn't disregard your claim at all...I have been refered to as a ricer in past by some friends...but I assure you, you can gain a little hp from sound...
I know that sounds ridiculous...but exhaust resonance has a lot to do with pulse pressure...stock mufflers are tuned for keeping resonance supressed...so it doesn't annoy you, your girlfriend, or your neighbors...and by freeing up resonance by means of only a muffler (no screwing with diameter, etc.) you can pick up small gains...Some mufflers such as the one incorporated into the RB cat-back (don't use this example extremely, that thing does have a .375" bigger ID, which makes big brunt of its gains) have baffles that are not much more "free flowing" than that of the stock one...mufflers such as that really only make the exhaust louder, because of the arrangment of its baffles...yet they do not do much for flow...and yet you can still gain a couple hp...this does not apply to most straight cut canister things...they usually do nothing on there own other make your car sound like a sh*tbox...and hurt pulse flow overall...
its all in scavaging and resonance...so the ricer adage that "louder is stronger" isn't entirely untrue...just a lot less true than what they say it is...