I am with Brentacre and they have like for like mod replacement.
I phoned up on Saturday morning and added £3k worth of mods and it cost me... £0.
An increased power limit to 280bhp is £130 extra (which was for 10 months at the time).
22 with 3 years NCB
Then again insurance is so personal that someone else with exactly the same scenario as me but who lives in a different town could be double the price of half as much or not covered by the same company... just shop around specialist insurers and insure your modifications LEGALLY.