I bought a black coupe from JDM here on the forum and he had made various modifications. Fortunately i used the same insurance company he did (admiral) - i thought it was simplest to phone admiral up and tell them i was buying a car from someone who was already insured by them, with modifications from standard, so could they transfer the listed mods for that car (and that car) onto my policy.
you'd think this was simple, but i did have to almost lose my temper with the lady on the line in order for her to do this!
anyway, in terms of cost i cannot recommend admiral multi-car higher, they insured me and my partner on an M3, a Renault Clio and a VW Golf for less than 2/3 the cost of Kwik Fit's renewal on just an M3.
JDM said they added £70 to his premium for the mods he'd made (quad exhuast, altered wheels and rear diffuser) and i phoned the other day as i'm intending to fit the AFE intake to my car shortly and told them of this modification, she couldn't find it listed so made a note on the policy.
34 year old male, no convictions etc, 12 years NCB, mid risk area, c£350 pa on a 3.0si with the above modifications. I think that's reasonable.