From one who has done insurance for over 40yrs, generally, insurers will not charge extra premium for modifications (unless you want to increase the insured value eg. Rims, stereo systems, etc). The question is intended for underwriting purposes to determine the risk, and therefore, for them to decide whether to provide the insurance and if so, on what terms. Providing the exhaust modification is legal, I don't foresee any issue. On my insurance, I have disclosed mods to steering, engine and wheels with no conditions imposed by insurers.