@danhenri The discrepancy you note is due to compression heating and the limited recovery coefficient of the temperature probe on the actual MU2. Based on the POH values I have identified the recovery coefficient of the temperature probe to be about 0.6. This is important for the following equation:
T_static = T_total * (1 + 1 / (0.2 * C_recovery * Mach^2))
Where T_static is the static outside air temp, T_Total is the indicated outside air temperature, C_recovery is the energy recovery coefficient of the probe. Modern probes are 1 but those are aspirated types on jets, on stuff like the MU it would be between 0.6 or 0.8 depending on type.
Regarding your performance issues, you may want to try out my FM modification, it gets the plane to within 1% of cruise POH values. I haven't evaluated takeoff or landing distances yet.