GTI TCRs are entirely different from production cars. What works for a TCR might not work for a production car. I'll have to dig for the emails from VW Motorsport and AP Racing, but the entire brake system is different. The TCR utilizes a fixed 2 piston rear caliper and adjustable bias.
Stock 272 rear setup is trash. PP 310 setup was/is a huge improvement, but still lacking. Floating calipers are the devil. Maybe it's time for a full HPA or AP Racing setup.
On the topic of master cylinders. There's a ton of misinformation being spread. In the US the 23mm master cylinder is specific to the Golf and Golf Sportwagen with 288mm front brakes. All GTI, R, A3, S3, Atlas, and possibly Alltrack(need verification) are equipped with 25mm master cylinders. It's easy to identify your master cylinder. The diameter is part of the casting. I have one of each, I'll grab my comparison pics off my hard drive. The brake booster is also specific to the master cylinder diameter.