I had a TMC box on mine, think it was the first 1.4 TSI in the country with it.
They are good options as can be taken out etc BUT if Seat really want to they could see the engine had been changed. Not via the flash count on the ECU but drilling further down they'd see more power going to the engine than standard. How likely are they to do this? Not really to be honest, it would be a lot of investigation.
Be mindful that you're talking about modifying your car, not telling insurance and then de-modding it IF you had an accident. This is an open forum and insurance companies are getting wiser to them.
Mine vRS made 211bhp with the TMC box (setting 7), ITG CAI and straight through centre section. I didn't have any complaints with it and it did it's job well.