The question you should realy ask yourself first is.....If i had a 2013 Mustand Shelby for 2-3 years and someone wanted to trade me for a Gen 2 GTS, would I. Firstly, the guy/gal that is interested in the trade would be all over doing the trade. Secondly, you would be doing either an even trade ( which your not doing now with the dealer, your adding money), or, you would be adding money to this deal as well to make it happen since the mustang in 3 years would probably be worth 35-40,000 and the GTS no matter what year would still be worth in the mid to high 40,000's. Thats in 3 years...in 4-6 years you would be adding alot of money to get your GTS back. Not to mention your novelty and feel while you drive that car would leave you feeling empty. No gauking,pictures or wow factor anymore.
As someone stated, go to the mustang forum and presume you wanted to do a trade for your GTS and see how many would jump on that deal over their mustangs.
662 Hp on the street, is not a bragging right. Driving a Viper is!! Trade your car in and get a good used 2008 ACR instead of the extra money your giving for the mustang and you'll get bang for your buck, IMO