It's not the Mach number but the air blast, measured by indicated
airpeed in the cockpit, that hurts you. It's also known as 'Q' force,
or ram air pressure. FWIW the SR71 Q limit is low enough so serious
injury is unlikely. OTH that's also why a lot of fighter e-seats have
straps to restrain one's legs and prevent flailing. As far as I know
all seats will fire on command; the ejectee takes what conditions
exist when he wants out. FWIW I personally know a man who ejected from
an F104A (lost all hydraulics at 35000 and 1.5) going almost straight
down at 1.3 M at 25,000. The Lockheed C2 seat with its leg and arm
restraints prevented him from receiving serious injury! OTH a lot of
F4 and F105 pilots were seriously injured at high IAS (550+) ejections
prior to the seats being fitted with leg restraints. Now I guess it's
only the crews' arms that get flailed . .i.e. bent back beyond
physiological limits by the high-speed air blast.
Walt BJ