Plugging in I think. Unless they look for it specifically, even a VW dealer won't pick it up.
Obviously if the car's in an accident, one of the first things they will check is is the car up to scratch (ie up to MOT standard) and was it modified in any way?
If you lie to the insurers and then get found out, no insurance company will ever want to touch you and at 17, that means potentially up to five years (I think) of no insurance.
Stick with the car. 138bhp is more than I had on my first car (74bhp).