About 15 years ago almost daily I observed traffic routing south - north over the London area indicating FL660 on SSR (there was no primary return) and which I assumed at the time were TR1s/U2Rs bound for Alconbury or Mildenhall.
I read somewhere that the (early) version used by Gary Powers operated at about this level, but you only had about a 4kt boundary between stall speed and mach buffet when this high, and he was brought down not by a direct hit, but by a missile exploding near his aircraft causing it to exceed these parameters - not sure whether it said he stalled or whether it just broke up due to exceeding its mach limit.