As mentioned earlier, the issue there is that there is very little data on the real aerodynamic behaviour of big jets 'outside the envelope'. For simulator purposes, most is just extrapolated from available data (see the NASA report on the subject).
Simulator manufacturers wouldn't mind, but they cannot afford the (highly instrumented and risky) flight testing needed to acquire such data....;
There are several companies that specialize in aerodynamic modeling - I will leave them nameless for now, PM me for details - that CAN extrapolate meaningful modeling (supplemented by wind-tunnel testing to refine) that will provide valid flight model extensions. I have posted several times about the ICATEE RAeS - see
RAeS Flight Simulation Group - Introduction to the ICATEE