Spot on..... If you fit the incorrect rating, you give your insurance a way on invalidating the claim.
I agree with you, about speed, but there is more than speed involved, as weight of car will generate higher tyre temprature, which is the same as running at faster speeds.... Although truth is, it is highly likely never to get to the limits.
However, insurance companies will alwaysbe looking to a way of invalidating a claim, and you would be handed that option to them on a plate.
Simple answer... Never go down the rating.