What year is your GT? If its the GT/CS in your avatar, then forget the Wilwood kit you linked, it uses rotors that are actually smaller then your currents rotors.
You are making this way too hard on yourself. There was a factory Brembo option. That Brembo brake package has been turned into an aftermarket kit by Ford Racing.
Ford Racing Mustang SVT Front Brake Upgrade Kit M-2300-S (05-14 GT, V6) - Free Shipping
14" rotors and 4 piston Brembo calipers. Its not going to get much better then that, and it is by far the cheapest option. There are even cheaper ways to assemble the same kit if you get resourceful.
If you want to go crazy, then look for the 15" rotor 6 piston Brembo brake kit offered by Ford Racing. Its the same front brake setup as the 2013/14 GT500.