Any mods that increase the efficiency of the engine (eg, freer-breathing intake/exhaust) will give better gas mileage ... if you drive the same before and after the mods. Most people aren't going to drive the same way after making performance mods.
For tips on getting the best MPG, see some of the hyper-miling sites and adapt/apply some of their techniques (eg, use gravity to your advantage) to your style of driving.
PS: If you drive it like a sports car, there's not a whole lot you can do to get better MPG around town. Other than quit driving it like a sports car.
