I can't speak with first hand experience, but give an insurance company an inch and they'll take a mile! A tyre not rated to the speed capability of your vehicle would mean an instant invalidation in the event of an accident.
I've never seen it in a mog situation, but I do know of a local lad who got completely hauled over the coals because the tyres on his Mitsubishi Evo were only rated to something like 120mph when he crashed it. The fact that he was only doing 50 and it's not legal to reach 120 anywhere in the UK had very little leverage with his case. The car is capable and it seems that's the end of it pretty much.