Well, if he says there is no need, then maybe he is willing to pay for any damages that might result after fitment. get him to sign something.........then see how sure he is.
I will always r****d ignition.........but retarding the 172 is not simple, there is no dizzy. And when you r****d ignition, you dont have full normal power when not using N20.
No need for progressive with 50bhp, but just check to see if he relys solely on the knock sensor.