Using the Rayleigh drag equation, and approximations for the air density, drag and frontal area of the pilot, and mass, at around 300 m/s (subsonic) a pilot might experience deceleration forces of about 40G. BTW, I assumed a drag coefficient of 1 since a Human being strapped to a chair is not especially aerodynamically slippery.
Now, if that was raised by another 100 m/s making it supersonic, how much would the force increase disproportionately?
Note that one person has ejected at low altitude supersonic and lived - just. So, how much of a difference does being on the wrong side of that dividing line make?