It is my knowledge...and experience...that a freer flowing exhaust most CERTAINLY will give a performance boost. Toss in headers, high flow cats (or none), and pure pipes and that exhaust will give a gain....pretty much over any stock exhaust. SLP Loudmouth and y-pipe. I had it. I used it. I saw the gain. Yes...there was a performance benefit.
There is no doubt in my mind that there will be a benefit from a chambered exhaust.
Wait.....hold on.....
http://www.gmmginc.net/Exhaust/index.php
Did that say "dyno tested...Improved Horsepower?"
More info on Toms stuff...
http://www.tomhenryracing.com/toys/ss_camaro.htm