I see it as quite simple really, I am asking the insurance company to take the risk of insuring my vehicle against theft or damage. Therefore I need to tell them exactly what it is they are insuring, particularly if I increase it's potential value. By modifying the vehicle say, by adding AeroMax alloys to the car. I can't see the insurer being prepared to cover the additional value of these in the event of a claim, if they weren't aware of them beforehand.
A lot also depends on the individual insurer and if they really understand the performance car scene. In my experience the specialist guys are usually quite switched on and not particularly phased by modifications, if these are discussed with them.
