If you advise your insurance company of a modification and that modification is not actually approved by the vehicle standards department, they can still crap on your claim. If you read these clauses in most (if not all) insurance documentation you will see weasel words hidden in there about how the mods have to actually be legal.
If I told my insurance company that I'd put a forward facing hornet scoop on my car, over the top of a tunnel rammed carb fed 600 cu.in V8, they'd say, "Thanks for the advice". It would not be any good if I had an accident where I scalped a pedestrian when the throttle cable jammed open in a carpark burnout and put the car into the crowd though. They would be well within their rights to point to the pedestrian maimer on the bonnet covering the non-emissions compliant, 3x too large engine and laugh at me.