You have to tell them about ANY modifications. Sometimes, you even have to tell them about factory fitted optional extras.
A modification is ANYTHING the car did not come with from the factory - so wheels, window tints, suspension, bodywork, lights, exhaust... if you changed it or added it, you need to declare it. Even if it's a part from another, similar model. So if you have fitted Cupra or LCR bodywork, it needs to be declared. The performance isn't the only reason those cars have higher insurance.
Insurance companies will do one of two things if you crash and claim - or somebody claims against you - and they find you haven't declared something; they will void your policy as you have not acted in 'utmost good faith' when taking the policy... or they will demand a higher premium and a higher excess before taking the claim any further.
If they void the policy they will also do everything they can to ensure you don't get, or struggle to get, insurance elsewhere. They will add all your details to the CUE database the insurance companies look at, so any future insurer will know you committed fraud to get your policy and will refuse to insure you.
I've just spent a week or so on a couple of Insurance training courses as I've just started working for an insurance company (3rd biggest insurer in the country) so I've had all this stuff rammed into my head recently
In short... it's not worth the risk. Insurance companies are getting to the point where they will investigate any claim where they suspect somebody isn't being 100% honest.