wow. What Mach number will they be at? You'd think that all the criticacl speeds would be converging.
Its about wing loading, span loading and adequate controls and aerodynamic devices , and the biggie Reynolds numbers, not to mention crew environment.
At 100.000 feet the indicated airspeed would be 88kt (100 mph.) for Mach I. Should be fairly linear at those altitudes. So they will have a an indicated of somewhere in the 45 kt (50 mph,) range to make 400 true.
That would give a stall buffet speed of around 25 kt. Probably got some interesting aerodynamic devices.
Spoilers for roll and maybe a Gurney flap.
My bet is its not that good but if it even comes close, thats pretty fantastic!
A guess but... close.
BTW there is enough O2 to operate a internal combustion engine fine...at Mach III.