Exactly as Mr Savage has said re the front assist. There are things you would get into trouble switching off, like using VCDS to switch off your DRLs because they are obligatory on all cars made after some date in 2012.
Front assist fitment and usage is not obligatory by law (it would be fitted on all new cars if it were), and unless your insurance has a specific clause in it that says you must have all safety related devices switched on at all times then they will not be able to void your insurance if you go into the back of someone with it switched off.
Even with front assist on, there is always the possibility of hitting the car in front, if that car has a more sensitive front assist system than yours and better brakes, or your sensor is dirty.
I'm not entirely convinced that the GTI dropping 5 insurance groups is entirely down to the inclusion of front assist. The MK7 GTI only gained 10PS over the MK6 and dropped 5 groups, the MK7 GTD gained 14PS over the MK6 and yet it is the same insurance group. Perhaps the MK6 was overly high on it's grouping vs risk. 220PS isn't exactly remarkable in the hot hatch stakes any more - it is the GTI's handling that makes it punch above it's weight. There are plenty of non-sporty executive cars with as much power that are grouped as low. You've also got to remember the average Golf driver isn't some 18 year old boy racer either, so I wouldn't group it as harshly as a VXR or ST3 based on perceptions of the average driver for each of those cars.
As per the turning the passenger airbag off and a passenger sustaining more injury as a result of it not being on, you'd lead yourself wide open for a civil suit, but almost certainly not a criminal prosecution.
