I don't know. 5 years ago, noone would have said to get over 400 hp out of a GT, and here we are with 420 hp. Times are changing. As long as they can do the CAFE standards (or whatever they're called), we're in store for some fun in the near future.
I'm sticking to my guns on this, I think 435 to 440 hp would be a more reasonable number. Even if there is just a 100 hp drop in weight it would still be a fairly significant improvement in straight line performance.