Why can such a small difference in thrust make such a big difference in maximum speed?
and both are aerodynamically supersonic capable

No, not really.

The blunt nose and rather thick wing with not all that much sweep of the Hawker Hunter are not suited for supersonic flight.

The engine inlets of the Hunter lack the (movable) shock cones like the Mirage, which are needed for flight at higher Mach numbers (either a movable shock cone or a variable inlet vane are needed above around M1.6), and the shape is generally not suited for supersonic flight. Supersonic inlet needs to create an orderly pattern of shock waves inside to maintain smooth air flow and the triangular shape does not look like it would do that.


Also the Hunter engine probably has lower exhaust velocity, which makes it more efficient, but limits the maximum power at which it can still produce thrust.