The 2015 Mustang GT will have more than 400 hp right off the bat, but the Ecoboost version will have the potential to meet, if not exceed that amount of power with some simple aftermarket mods ...
This is no longer the 80's, so the question of "V8 vs. 4 vs. 6 potential hp" no longer exists.
It all comes down to the exhaust sound. If you want a V8 rumble, get the V8. If not, get the 4 or the 6.