Nice.
[trying to stay on topic]
My stock muffler weighs ~ 45 lbs.
I'm not sure how that compares to most stock mufflers, but I can save a significant amount of weight (20 to 30 lbs) just by switching to an aftermarket muffler. The weight gain would probably only help city mileage.
The other thing that I've been looking at (I don't know if others have actually dealt with this) is that switching to a different rear exhaust might make cleaning up the aerodynamics of my rear a bit easier.