Are you sure you're not obsessed w/ backpressure ?
Not sure what your real question is, but here is a stock muffler mod that makes it a little louder.
I would assume as the holes were made bigger, it would get louder, and that back pressure would get less.
For myself, and IMO, this mod was easy, its louder, didn't cost a dime, doesn't sound bad, and can easily be brought back to stock. As far as back pressure is concerned, it doesn't concern me in the least.