I declared all mods on the B, dont think it cost that much extra in the long run and it gives you piece of mind. The C I only have the tuning box on and think that cost an extra ?20 with my current insurers (cant remember who that is with though)
Hopefully in the future, Chris Knott will be able to sort out the insurance as I have been in talks with Tim Slater from there and he is going to sort out a clubvec discount (passed over Chris's details also, so hopefully you guys will benefit too)
Other insurers known to be good with modded cars are Adrian Flux, they do club discounts also :)