Yea, I know, unconvincing name and it's new to me.
Recently I contacted a company in Italy after reading a thread in Corvette Forum. They create big brake kits including for, ~tada~, Corvettes. Out of curiosity, I asked if they made any for an Opel GT and they did. The merit of their calipers is their low profile, as in, their calipers are smaller shaped than those of Brembo. This feature allows smaller diameter wheels to be used and wheels with less lateral, inboard space. According to them, all their brake kits for Opel GT can fit behind a 17" wheel.
There are four part numbers for an Opel GT, one is for the rear and it includes 330mmx12mm rotors paired to the OEM calipers. I assume the rotors are solid and adapting brackets are included.
Keep in mind the following are for the front only. The "meekest" kit includes 6-piston calipers and 314mmx28mm one-piece rotors. Next up is 10-piston calipers and 330mmx26mm two-piece rotors. The big daddy uses larger 10-piston calipers with two-piece 340mmx26mm rotors. The funny part is all these use the same part number for pads.
The smallest front kit is priced at €1450 including shipping, again I assume, from Italy.
Click here for Tarox website. The price seems cheaper than Brembo aftermarket kit but more expensive than Brembo OEM, i.e. used in HHR SS and Cobalt SS. I have an Excel spreadsheet containing part numbers and minimum description of their brake kits for Opel vehicles but I don't know how to post them.
So there, another possibility to spend money on a Kappa.