You tell them.
I can see your reasoning, and in truth it's how it should work, however when you sign up to insurance you agree to tell them of any material change that would effect how they class you as a risk. Improving the brakes means you can go faster, which increases your chance of a bigger crash. It also makes your vehicle worth more, which increases the chance of it being stolen. You signed a legally binding document, and deciding afterwards which bits you do or don't feel like complying with doesn't really work.
People are paranoid for good reason, as no-one wants to be left with a bill in the hundreds of thousands of pounds in the worst case scenario. If you're so sure that your insurer won't care, and are happy to argue that point with them, then why not tell them?