When i first passed my test i had a MK1 Clipper Cab with the Wide arch Karmann kit. It had aftermarket alloys, a straight-through pipe, lowered over 60mm, the works.
Stupidly, i never told my insurance any of it. Luckily i didn't prang it but still some jealous toerag slashed the roof to get at the CD player, and i had to fork out £400 odd for a new roof. I would never ever do anything to my car again without telling them about it. It just ain't worth it.