What happened to this experiment? Did you ever try the S550 base Mustang GT brakes on an S197?
I'm interested in this idea as the base S550 GT uses the 4-piston mystery calipers 13.9" rotors and 18" wheels. I just bought a '14 GT non-performance pack with 18" wheels and the base S550 calipers/rotors might be a cheap way to get better brakes on the car and not have to upgrade my wheels.