I'm with Hagerty, Lee. Most of my 'fleet' are standard, but they were happy to insure my Lotus Elan with Cosworth BDA engine (at 70% increase in power over standard) and a super-low miles Jag valued at 5 times a normal-miles one. One policy for quite a few classics, including the Vees, and nice and easy to work with.
They are superb on claims as well, and show that they really understand rare / classic and modern classic cars. A mate had a fire in his concours Escort Twincam, and most companies would have written it of. It has a lot of parts in the engine bay that you can only buy secondhand, and then for a daft price, like an aircleaner setup for £2000. They told him to get an estimate on getting the bodywork sorted, and to itemize the bits that were damaged that couldn't be bought new, giving the price he expected to pay on ebay.
Two weeks later a cheque for £25k, the full amount of the estimate, arrived with a little note wishing him luck with the restoration! Not many insurance companies would trust the car owners so much or bother going to the trouble of accommodating such a claim.
Mark