One of the more annoying quirks of MSFS is how it (mis)calculates indicated airspeed from true airspeed. Unless you do some tweaking in either the .air file or in Aircraft.cfg, your IAS will be incorrect. The recommended way to deal with this in FSX is by tweaking the airspeed_indicator.n entries in Aircraft.cfg, which will set a scalar value and a base offset value for IAS calculation.
To my horror, I just discovered that in Draken (FS9 and FSX versions) I had set the airspeed_indicator.0 values way wrong, resulting in indicated airspeed in Draken being about 100 kmh too low. This of course negatively affects things like flight plan calculation and formation flying. Furthermore, since I have calibrated the alpha incidence angle (aoa) curves in the FDE to IAS rather than TAS, the whole envelope is slightly off. This is the main reason why I have failed to get realistic performance at high altitude.
Changing airspeed_indicator.0 and cruise_lift_scalar to the following values were successful in my case:
airspeed_indicator.0 = 0.93, 8.0
cruise_lift_scalar = 1.9
If you feel like experimenting, use these values as a starting point. Please note that other parameters (lift and thrust scalars) may also need some tweaking to get realistic performance.
Thanks to Johan at Novelair for reporting the airspeed error.