RE: Modifying Mac' One-Piece Mufflers
I read in the past of someone who was drilling them out and getting more revs, possibly someone in combat. Being highly tuned does not seem to be the reason for their popularity. Rather, they are light and sturdy alternatives to the standard mufflers that tend to come apart and loose pieces. On the other hand, I think some folks do report a couple hundred rpm increase over stock mufflers.
Ernie