Flow and noise are directly related. If you want flow, scrap the muffler and get a straight pipe - but it'll be noisy as all buggery. A big fat muffler will kill the noise, but kill the flow too. Basically the more you have in between the engine and exhaust tip the quiter it'll be.
Turbo's get away with cannons and the like because the impeller wheel up close to the engine mutes a lot of noise, but NA's don't have that, so a turbo spec muffler will make the noise problem worse.