GTM Brakes

I dont have a good answer. I am still building my car.
I have not seen/heard of anybody running a booster, so I would assume that setup is good for the stock C5/C6 calipers.

I went a different route and upgraded to Cadillac ATS Brembo 4 pot calipers with Corvette C6 Z06 rotors all the way around. This is sort of a bolt on setup. The calipers and rotors are a direct bolt on. I kicked my Master Cylender up from the 3/4 to 7/8. Did this because of the increased caliper volume, with the 3.4 in MC's the pedal was a little long.

If your pads are soft and street biased it's going to feel pretty normal. If they are hard pads for track that need temp, first part of a drive can require harder push on the pedal. For significant brake performance, use the AP Racing Corvette Radi-cal brake package. PM for sizes front/rear for GTM as it's not exactly like a GTM.

Also, for anyone building a GTM today, in 2023, make the minor effort to install Teves Mk60 ABS. All it needs is 12V power, ZR1 hubs and you have amazing ABS.

The Corvette brakes are PBR's similar to the Cobra brake setups and they work OK with decent pads. Like the roadsters I really can not recommend a booster in the brakes because of the lightweight of the cars. My Wilwood kits are a big step up in braking and a designed to be a performance manual system.