Is there any definitive reference about what modifications / additions should be notified for car insurance? I always assumed that performance related upgrades were relevant but I have seen comments that just about any undisclosed changes to the car could void insurance.
It used to be the case that insurance companies operated on that sensible basis and didnt wriggle in the way you now suggest. But sadly, customers mostly buy insurance only on cost so companies compete on cost and that means being sharp about claim handling. However, this only comes to the surface when there is a decent sized claim warranting investigation by the coimpany or their loss assessor. When you take out insirance only the most rudimentary checks are done - the fine print is examined at claim time.
What this all boils down to is that the wise man infomrs them in writing of absolutely every modification when he takes out the insurance. Usually it wont affect the cost at all, but it will covber your backside if there is a big claim. All according to SWMBO who spent her working life in insurance.
