I thought speed rating had everything to do with simply the tires being able to stand up to all of that centrifugal force (centripetal force? I never remember which one is right)

your lateral acceleration is V^2 / R and if you're going 150mph (67meters / second) and assuming your tires have a stock circumfrence your radius is going to be .315m your lateral acceleration is 14,231 m / s^2 (one G is 9.81 m / s^2) so this is going to be roughly 1400 times as strong a outward force on the tires as gravity exerts on us. The only way that I can think of being able to take this much stress is if they are very light and have very strong re-inforcing materials in them.