Current value of the mods isn't like for like.
For example, your turbo kit may have cost you £10k, but on the second hand market it's only worth £5k. As such, if you write the car off you're only getting £5k back. All figures for illustrative purposes only. At least, that's how it reads to me, IANAL etc.
Given the value and work that's gone into your car, would you not be better off with a guaranteed value policy?