I found that after I installed a Borla performance muffler, that my mileage was somewhat increased. Probably not so much from being less restrictive, but because it was so LOUD! I found myself accelerating MUCH more slowly just to keep the noise level down.
Since then, I've installed a Supertrapp sliencer tip & it seems much better, though I haven't been able to take it out on the road since it's in the shop having a new r. output shaft installed & won't be done till nxt. wk.
Seems to be the price we pay to be VXR'S.
Ldub