i'm with Norwich union.
when i told them about my Modifications, they didnt even raise an eyebrow.
the chap said i can have "UP TO 3 MODIFICATIONS" with no additional surcharge on my premium.

but they're not interested in covering my ice install. they will only pay out upto £500 for ICE/stereo
their service is TOP notch, my golf Had a misfortunate ending, so i've been through the processes.
i dont think anyone will notice that a car has been remapped Mr HDI FUN, because it would be a very long winded process, they would have to download the data on the ECU an compare it with another data map from anothe car EXACTLY the same as yours. and Manufacturers often change specs/ hardware suppliers etc over the years of building the same model.
so it is very difficult.
in my experience
the insurance engineer walks around the car for the condition, looks under the bonnet, wheels, and scrutinizes the accident damage and then checks the mileage to put a value on te car..
Nobody checked the ECU on the golf.
Remaps dont really leave any evidence.
i would advise everyone to tell them abot any other Modifications though.
and like Waynne said;
be careful to discribe your Modifications in their best light.
Coz Insurers like car enthusiasts more then Boy racers.