I have 13 model, track package, 17k on clock, instake, exhaust (corsa), LMS 93 octane tune, lowering springs, TSW nuburg (huge weight savings), and 245/45R20 DSW's.
I do about 1200 miles month for work; lot of highway; get around 21-22 mpg. Close to about 400 miles to tank.
When I drove to Texas in November (1600 miles one way) I got 25 mpg with average cruise set at 80mph.