I have flown the DC-8-62 at FL410, there was no problem with "coffin corner".
Handflew Learjets 24/25 routinely at FL450, I believe the margin is somewhere around 40kts. Flies nice at those altitudes. In testing I believe they did steep turns at those altitudes.
NASA used to fly their 23 to FL500, they also took a DC8 to 450 and did a highspeed dive attaining M 1.07. First supersonic tranport??