A little wing (To improve looks, and it adds a little bit of downforce):
Some "Shelby" lettering in the back, and "GT500" on the side:
side vents on both sides of the car:
Bigger Hood Scoop (For a better looking.)
Bigger Brakes (For better stopping perfomance)
Better quality tyres (It drastically reduces the wheelspin of the car, Making it easier to control.)
Decreased 0-100: now it's got a...