You are not the first issue with the brakes on a 2015 Mustang
by far. Search some of the other Mustang forums, pretty much every forum has a few people complaining of squealing or grinding sounds from the front brakes, and Ford is trying anything from different pads, new rotors, to new calipers.
Sounds like Ford has a quality problem with the pads or calipers on the new 14" (inch) GT brakes. On the flip side, to date I've not seen any complaints regarding the base EcoBoost brakes or the GT Performance Package 15" Brembo brakes.
Aren't first year production cars fun...