I recently saw a repeat of a "Power Block" TV show... Horsepower?...not sure, where they tested several mufflers on the same mule engine on a dyno. I think they might have used a LS-engine. Anyway, best to worst was a range of about 8-10hp max. improvement vs. stock muffler (on a street/strip build).

70 Roadrunner convt. street car 440+6, NOS, 4-spd, SS springs '96 Mustang GT convt. street car '04 4.6 SOHC, NOS, auto, lowered "Officer, that button is for short on-ramps"