I lost 2 mpg overall going from 3.55 to 3.73 with a AT. I drive 50-50 hwy/street. RPMS increased approximately 250 at highway crusing. Nice gears and I'd do it again.
Have you thought of 18's? I run 265/50/18's up front and 295/45/18 on the rear, lowered with Eibach Pro Springs & D-Specs, with absolutely no rubbing. Just thought I'd throw this out there.