First, SF1 and SF2 data files are not interchangeable....
apart from that.
There are aerodynamic coefficients, more specifically the drag, which is set to such values that never allow the plane to break a certain value. Even if supersonic drag is low, because of high subsonic drag, the plane can never break the sound barrier. This is a method to create subsonic-only planes (like Yak-38 for example)
You can find a list about these values. As FM's are usually editted the simplest way, usually this has been done with CD0 = Zero-lift drag coefficient
CD0MachTableNumData=13 CD0MachTableDeltaX=0.20 CD0MachTableStartX=0.00 CD0MachTableData=0.984,0.9955,1.000,1.052,1.676,4.589,1.232,1.129,1.107,1.050,1.015,1.016,8.145
This means, the table has 13 values, each value means 0.20 mach speed increase, from zero. So you can count, between 1 and 1.2 machs, the CD0 is 3 times more, and at 2.8mach 7 times more. So this plane, depending on engine power has low acceleration when breaching 1Mach especially at low altitude - and it cannot break 2.8M at any circumstance. (it's a Flagon, and it cannot reach that speed anyway with the current engine power) So take a look at this value at each aerodynamic part of the plane, and experience with them. But keep in mind setting realistic values needs proper skills (in physics for example))) and tech data, too.