Interesting so far - on the subject of 295/30 they won't fit on standard rims as too narrow 10" minimum. So its KU31 275's or KU39 255's, the KU39 is the latest tyre and slightly better and potentially no insurance issue BUT
If we are suggesting changing tyre size could invalidate insurance how many people have informed about not using run flat as they were OE and changing tyre manufacturer as well?

We could go to silly extremes such as using Rinkydink tyres with stopping distances double that of premium tyres - my attitude is that if they are and approved by a manufacturer such as Mercedes (Kuhmo) that should satisfy the insurer and also if the tyre size is a correct size for rim and causes less than 3% change in circumference then also OK (275=2.12%) and don't forget that the rolling circumference changes anyway as the tyre wears approx 1.6% in case of 255/35/19 from new to tread depth marker.

Would be interesting to get an insurers opinion on tyres remembering they had objections to people fitting safer winter tyres in the winter.