I have a Borla "turbo"...& still get pretty poor mileage, but I've committed every other "mileage sin" that there is. (big knobby tires, lifted, roof mounted spare, supercharger, & the list goes on...)

Also, when first installed, it was a bit too raspy/ricey sounding for my taste in audible exhaust...
After experimenting with a Supertrapp tip (too restrictive) to tone it down, I finally settled on an inverted cone to take some of the rough edges off the exhaust note.