I hope your going for sound because that's all you'll get. This car already has a high flow 2 1/2 inch exhaust, one of the reasons they are putting out the HP they do. The Mach with a full exhaust is lucky to get 15-20RWHP and the GT will be FAR less if any. Spend your money on real HP mods IMO this is a big waste but if your wanting to hear you coming I guess it's a OK mod.
BTW a tune will bring much more RWHP than any exhaust. These cars are running rich and I'd like to see the before and after dynos because they are probally leaning the cars out a little to achive a HP gain. A 13:1 ratio is what your after with your car.