The lead designer when the 2013 GT500 motor was announced said multiple times in his interviews that the Trinity motor was going to be the most powerful ever. Meaning in history and for a while to come. Look at how the 2003/4 cobra was made. It took them until 2007 to bring out a car with more power then that.
Also, all of these manufacturers (Ford especially) arent going to be going nuts with horsepower battles with the CAFE standards in the USA, there are some strict guidelines coming down the pipeline in the USA with emissions and fuel economy.
Its very possible that you might see a car with a DCT and turbos being quicker to 60 or handling better in the coming years, but you may not see more then 662hp from the factory again period. I could be wrong but I dont think its likely.
Actually, don't be surprised if you see hp dramatically increase! Hybrid sportcars are going to be the future and probably next on the list is the 2016 GTR. The rumors around it are saying 800hp with basically the current drivetrain and the addition of a 200hp KERS.
I would put money that Ford is looking into the same thing as we speak for the Mustang. Hybrid systems is the obvious route as they are in 3 major hypercars right now and automakers are going to be looking to capitalize on that to offer the average buyer a taste of that driving experience. It's best of both worlds really. There's no real good reason to not experiment with it for high performance use.