Edley, the mods like the exhaust actually help the fuel milage, but the more radical you go with it, ex. a rear gears or cams, will make it suffer a lot, depending if you have the V8 or V6. Most of the little odds and end stuff like exhaust and air intake will improve it, while giving you more power. By the way UT, I tried and tried to make my dad's Mustang into a doner car for one of the new FFR kits, but he didn't like the idea I came up with.
The only thing that I don't like about his car is that is an automatic tranny.