I think if you chnage your original standard muffler to 'true' performance muffler, it should have some very slight performance increase, as I read in HowStuffWorks website, mufflers tends to give "Back Pressure". So, if you installed the wrong muffler, you could exprience loss in power...

I guess that explain why certain "Ah Beng"s cars sound so loud and noisy but the car still there... Even those cars like Evos or Subarus with bad modification, the sound dem loud (uh, it's noisy and not nice) and the car like no power.