If I recall correctly, in the US you insure per person, whereas in the UK its per car. So its pretty much mantra here in the UK that if the car is modded you tell the insurer (indeed its a question on the insurance application forms)...
That indeed is correct by person and registered vehicle. However, if the vehicle is modified beyond stock and those mods cause changes to performance and safety then the insurance company can withhold payout, its sometimes written in the very small print that no one reads. It's a grey area but something that is gaining more prominence.
