Ive always told my insurance company about mods, and never been that bad. Although i dont tell them each time i do a mod, just in stages, as admin charges are about £40 a time!
Last car was a E46 330 and it was £370 FC and i added the following
Alloy wheels
suspension upgraded
sports bodykit (front, side skirts and rear bumper) or cosmetic mods
exhaust
remap
air filter, or induction kit
well and a few more, but by the end it went up to £420
By the way, if your going to mod your car and tell them, then go with a company that does not hate mods!