Stang TVStrength posted an article where an exhaust company did a before after thing. They showed a big improvement in power. Personally I think some trickery was involved, but rear wheel numbers are in the ball park of Ford's claim of 435hp.
pretty worthless test...no way is there 32 hp in just a cat-back...
http://www.stangtv.com/news/video-first-2015-gt-with-aftermarket-exhaust-and-dyno/