Roush Mustangs, at least in previous years, are indeed just a body kit and stickers. They may have some suspension work but don't expect any real performance difference until they add forced induction -- and double the price of the car.
Same thing goes with their current F150. Even with the supercharger they're mid-high 14s even with 400 or 450 claimed HP.
Saleen is the same way -- nothing but stickers & urethane parts until you pony up the cash for a higher model.