This whole insurance thing is quite surprising, I was chatting to a broker friend of mine and asked him how these mods could affect your insurance so much, The short answer was, the way the insurance company look at it is if you make a claim they have to pay every claimable part of the car insured in there eyes,,
So I then mentioned I have £1500 worth of up graded audio and have bought a hardtop, the answer was if you dont tell them you techinacally cant claim for those items (likewise if I had a £1000 guitar in the boot, which I do sometimes), but it would not affect your claim on anything standard, and in most cases they would not bat an eyelid anyway, and would most likely cover them (not the guitar obviously

).
One of the things he did say, was the mods that can affect your claims and what an assessor looks for, are things like, non standard wheels, non standard brakes, engine modifications, non standard suspension etc,
Rusty