The single-seat MiG-25 could achieve high speed, altitude and rate of climb; however, it lacked maneuverability at interception speeds and was difficult to fly at low altitudes. The MiG-25's speed was normally limited to Mach 2.83, but it could reach a maximum speed of Mach 3.2 or more with the risk of engine damage.
My guess is that the inlet ramps would not close enough (reverses venturi effect) and the speed of the air at the turbine blades is supersonic and not subsonic.
The supersonic air could cause turbine blade damage or possible compressor stalls at high mach.
The deformation of the airframe that you mentioned would most likely be damage to the radome.
It can technically fly faster than Mach 3.2.
If you have been in the cockpit, there is a fuel temperature warning light.
You are supposed to drop speed once it is on.
I think Belenko made up the engine story for se reason.