In order to void your insurance they have to prove that the "illegal mod", provided you've also listed it on your policy, contributed to the accident.
Lets use a dodgy catch-can as an example:
Could it contribute to you running up someones arse?
Highly unlikely.
Could it spray oil vapour on to your exhaust housing, or a shorting wire, giving you a nice little bonfire?
Sure could, and if the insurance company can prove it, then you're in trouble.
But an internet forum isn't the best place to get this type of info, give your insurance comapny a call, don't mention your policy or anything just make "general enquiries".