Car insurance is a nightmare and what they charge for modifications can vary a lot from company to company and not always in a sensible manner. It is also bizarre what can affect the premium that most people would not consider. For example: adding stripes/decals (not generally a problem for Z4 owners but often done on minis); up badging such as adding a cooper S on non cooper S or Z4M on a non Z4M. Some of the interesting ones I have come across £3.50 increase when I fitted larger cooper S brakes on a standard mini; £10 decrease when I fitted 80% tinted film on rear windows.
Then to a really annoying one. My daughter was hit by a 17 year old driver who lost control of his car when turning into a side road. My daughter was stationary at the junction waiting to pull out. He then proceeded to drive on and park up about 500yds away, but a passenger saw where he had driven to so daughter followed him. He was clearly over the limit so police were called. He was found to be 3 times over the limit and had borrowed a car from a friend which he wasn't insured to drive. We ended up settling with the guy without going through the insurance. None of this was in any way anything to do with my daughter, but since then all insurance companies increase their quotes. The worse I found was one company increased it from £700 without declaring it to £1100 for declaring it. Her own insurance company increased her policy from £330 to £400.
I find generally, give insurance companies a chance to increase your premium and they will. However give them a chance to get out of paying up and they will too, so I always declare everything.