Us UK owners are in no doubt that undeclared mods can easily invalidate cover in the event of a claim of some kind. Long ongoing argument generally about whether to declare but it's not as scary as it needs to be, and depends on the insurer. I modified my last car quite comprehensively and when renewal came and I claimed all the mods (including gauges etc., everything down to the very last detail, and I modded it just before renewal

), it only only went up by about £200-300, which I didn't think was bad at all. Basically, you tend to pay the hike for it being modified, then when you add anything else, it may or may not need a daft tenner adding for the policy. Long and the short of it is, worth getting as many quotes as possible from known mod-friendly insurers before modifying the car to see what you'll face. Many offer club member discounts, this helps and means they appreciate the fact that just because the car has x BHP, it doesn't mean you're going to hoon it everywhere, in fact you're more likely to take extra care of it than you're average point-to-point Mondeo driver.