Mitchs caliper spread was verified w/ a veneer caliper. the open side was wider than a new unused GM set and the open side closest to the center of the rotor was wider than the part nearest the bridge of the caliper.
now, i've run the very same caliper since i started. havent even done a rebuild on them. i also dont run cooling ducts. Mitch and myself run the same pads, so we are at a loss as to why my live and he has spread one set on his CMC car. cooling ducts would almost surely cure this issue. that and the use of C5 calipers will help.
as far as 4 piston calipers being legal for CMC-2, i dont like it. 13" 2 piston brakes are plenty for a CMC-2 car. in fact, i predict that the stock mustang brakes for CMC-2 will be the most common set-up for the ford guys.