I've also wrestled with the tyres dilema.
I had heard, and seen echoed in this bulletin board, that tyres should be rated for the maximum speed of the car. It sounds sensible, although a 150mph tyre on english roads is unlikely to see more than 120 /130 mph, unless you were very brave/reckless, and I guess insurance companies may 'require' the higher rating on the tyres.
However, the original recommended SUMMER tyres are rated at 150MPH, while the original spec WINTER tyre is only rated to 130MPH.
Anybody disagree?