was this project mostly cost driven as it seems that the calipers are pretty much identical to the cobras although i've heard that the C5s are stiffer... hoping someone can chime in.
C4 corvette PBR: 38mm pistons (x2)... this is same as the 94-98 cobra PBRs
C5 corvette PBR: 40mm pistons (x2)... this is same as the 99-04 cobra PBRs
99-04 GT PBRs used 44.5mm pistons
Few reasons why I went this route. Some technical. but mostly just to do something different. My original Mach 1 13" setup was pretty beat up from autoXing years ago, and then sitting 10 years. It needed to be refreshed, and I wanted the car to look good. I didn't want used Cobra parts, and NOS bullitt calipers were VERY pricey. I've also always been a sucker for good brakes, and always looking to upgrade even if it was very minimally.
In researching how to do this, I read a number of sources claiming the C5 caliper was stiffer than the C4 caliper, and the C6 caliper even stiffer. Also saw the claim of 30% more pad area, covering more of the rotors surface, and that translating to more even rotor temps due to the pad sweeping the entire face...something I never liked about the cobra setup only touching the outer edge. Also, which might be the biggest thing about them, is that they can accept a 1.25" rotor, while the C4/Cobra caliper can only fit a 1.1" rotor through the channel.
If this was a track car and I didn't care about looks, I would have simply resurfaced the OEM 13" rotors (still plenty of thickness), bought the Baer caliper abutments ($125 each) and then got a set of used 05-06 GTO calipers off ebay for ~$100/pair. I'd probably research a 1.25" rotor for the the setup as well.
But, I also added things like the X-drilled rotors, which I'd never put on a serious track car, and the red Z06 calipers just to give it a bit of visual pop and controversy. I did the rears to match with powdercoated red Cobra calipers. I've put 100 miles on the setup just cruising around. More than enough brake for street use, and I find it hard to actually get them heated up due to the little pedal effort require to actually stop the car from 40MPH. It's also fun when people see it and comment "How did you do that?"
Here's how the car sits today. Wheel weight issue was resolved. No problem there. Car stops well, and combined with the 03-04 Cobra rack upgrade and solid steering shaft, it's VERY fun to drive these days.
BTW....17" Cobra spare tire does NOT clear these brakes.