Has anyone seen this modification? I don't recall it. My garage is littered with C5 calipers...C5 base and Z06 calipers are identical (other than the color) BTW...
I will preface this by mentioning that I will have ABS, but I've torn out the junk C5 EBCM/ABS for a motorsports grade ABS system, probably the BMW MK60.
With the mid-engine layout and rear weight bias, I don't like the idea of the small single piston C5 rear calipers.
Looked at options including big brake kits. But I don't think the 2600lb GTM needs a big brake kit. It just needs more rear brakes.
So I'm installing the C5 stock front calipers on all 4 corners. One benefit of this is retaining the parking brake, that you would lose if you don't keep the rear C5 or C6 rotors.
To run the C5 front caliper on the rear, you only need a C6 rear rotor from the z51 or j55 package (this is completely compatible with the C5 parking brake which is a drum brake inside the rotor!), plus the C6 j55/z51 front caliper bracket to position the caliper appropriately on the rotor.
If I find that I have too much rear bias, I'll start with a more aggressive pad up front.
Worst case, I go bigger up front.