From my experience, flat fronts seem to be able to run thermos, but when i put one on my RS (standard engene too!), it overheated at speed, but cooled itself enough at traffic lights...
Took it to a cooling specialist, and he said it could be the design of the front end - where air come in from under the bumper, and need the engine fan to pull in some air while moving. Anyway after pulling the thermo off and putting the engine fan back on, I have had no other issues in 10+ years since....
Both my flat fronts have worked engined and thermos, but do not really overheat, I it has to be the design of the front end.... (And yes I did check the thermo was wired in correctly, and the same one on the autocross car does fine on the flat front!)