I just checked in a whole bunch of backlogged fill-ups on my Fuelly account, so figured I'd contribute. The below includes my track weekend and a whole season of autocross. I also coach a sport part time so there are frequent short trips, and my car is filled with an extra 50-100lbs of equipment, plus my 245lb ass.
Tracked Miles: 11,406
Average MPG: 32.6
Best MPG: 37.6
Average Price per mile: $0.087
My mods that may affect gas mileage: CS Catback, CS Shock/Spring Package, Stock sized snow tires in front for the winter, 205-50-15 summer tires on lighter 15x8 wheels. This is the only car I've ever owned that every mod as made me enjoy the car more.