In the 60s I flew the Canberra and on one occasion got to 53,000ft dressed in the same kit I flew in at 250Ft. It was definitely designed for 250ft.
At that great height MCrit and stalling speed were close neighbours and the biggest problem was not staying up there but getting down. If you moved the throttles you risked a surge, if you turned you encountered compressibility. I was so busy thinking I was going to fall off the knife edge it never occurred to me what would happen if we lost the canopy.
Thank God theTornado could only go to upper twenties - and only to get more petrol.