does an aftermarket muffler only, increase MPG's or performance in any way, or are they only good for the sound?

 

It will improve both if you upgrade your intake too. The exhuast can only push out what it pulls in.

 

I recently bored my throttle body and installed a Dynomax cat-back with the Super Turbo muffler. Mine definitely gained some pickup when pulling out onto the highway and climbs into the power band more easily. No noticeable change in low end power or mileage. Definitely worth it to me.