Without some pretty large changes I don't think the Mustang has a whole lot to offer in the Horse Power wars than the last GT500 already did. The problem has already become an issue of traction. More horsepower doesn't help. Even with slicks on the back as wide as the Shelby would allow, you were still breaking traction in almost all gears. They've also pretty much topped out in the top-end speed department too.
The next improvement needs to be a major lightening project. Otherwise you're not really going to improve the performance of the vehicle very much. Smaller engine with tweaks to make the same horsepower, aluminum body panels, and a general lightening of the whole vehicle. Cut 500 to 800 pounds off the vehicle. The only other option to improve performance would be all wheel drive, and I don't see that happening.