An exhaust change is not going to do anything for your mileage, but it CAN change the sound. A couple of the other Mods on this forum (cleatus12r and 88Racing) know more about exhausts than me though, as does Bill. I'm really only "parroting" what I've learned from them. (I think I've got it right.)
If your ties have a more "aggressive tread" they probably have more rolling resistance. Cold inflate them to the max pressure shown on the sidewall. That may help some.
- Jack