I've found the drag curve in supersonic flight to be remarkably accurate compared to textbook material I have access to.
What happens with supersonic planes is you get a "Mach bump" - a region of maximum drag. The exact speed of the Mach bump depends on the plane's overall geometry, but basically goes with the inverse cosine of the wing sweep angle. Beyond the Mach bump, drag decreases to some minimum, and then gradually builds up again in a normal square-law curve.
The big problem in X-Plane is engine performance. In supersonic flight, the air is compressed as it decelerates to subsonic speed in the engine intake. As the engine gets denser air, the thrust increases. For a considerable range above the Mach bump, thrust increases faster than drag. That's the fundamental reason for the apparent runaway acceleration. But this is a real phenomenon.
In real planes, the geometry of the intake can accomodate some range of Mach numbers; beyond that shock waves start hitting the compressor face and all hell breaks loose. This is why really fast jets have variable intake geometry. X-Plane models this in a crude way with the engine mach limit. Near the Mach limit, X-Plane simply rolls the thrust off and you run into a wall.
What's completely missing is temperature limits. As the intake air is compressed, its temperature rises (simple thermodynamics at work). As an example, when the SR-71 is at its Mach 3.2 cruise, the air temperature at the compressor face is 800 degrees F. The engine has a fixed maximum temperature it can tolerate at the turbine, so as the intake temperature rises, there is less headroom for heating the air with fuel. As a result, as the temperature rises the engine performance drops off. For many supersonic jets, temperature is the real performance limiting factor. The afterburner allows you to get around this temperature limit. It heats the air after it's passed through the turbine, where the tolerance for heat is much higher.
About all you can do for now is set the engine Mach limit so
plane tops out at the right speed. In most cases, you'll have
high low altitude and non-afterburning performance because of the lack
of temperature limits.