I've heard of a few non specialised insurance companies refusing the insure people because of re maps or power related mods.
When I add mods to my car, it's going to be very expensive to add everything because unfortunately I'm only 20 and not many specialist insurance companies will insure me - saying that though, rang Brentacre before I got the car and they quoted me 2k for fully comp with like for like cover on the mods. I thought that was very reasonable for my age and 1 year NCB.