[whatthe]

I would try to build a custom header if you really want to help things out. It does start getting into period of valve opening, wave tuning, etc..

As these guys have said, the backpressure from the factory muffler is not likely to be a concern at cruising speeds and I would expect little gain to be had for any muffler at fuel economy rpm/throttle levels.

If it's turbocharged that's another story.