The 2.0 litre tsi engine in the Golf R has 300 ps as standard and is readily turned to 360-400 ps without any problems and easily capable of 500+ ps still being reliable. If the 1.0 litre engine is made to the same level 140-150 ps should still be reliable.
Personally I would like to hear from someone that is using a tuning box or had the 1.0 tsi engine remapped before I would consider getting it done.
With regard to tuning the engine and warranty if Seat find out you will lose your warranty for the engine, gearbox, drivetrain and associated parts but retain the rest of your warranty. That is why tuning boxes are popular with new cars, as far as I can tell they aren’t detectable as long as they are removed from the car before going into the
dealership.
I have a stage 1 remap on my 140000 mile 9 year old VW Touran and it has transformed the car making it so much nicer to drive with a greater spread of power across the rev range. It even made the engine feel smoother. I had the car remapped as the turbo needed replacing due to carbon build up so not worried about the turbos life span as I will get rid off the car in another 4 years or so.