We retested and as it turns out, the weather had apparently been set to create some icing that caused a problem with the airspeed indicator.
We found no problems with the performance in our testing, here are the results:
Altitude: 41,000 feet (12,500 meters)
Fuel: Center2 (rear fuselage) tank empty, all other tanks full
Top speed, normal power (full throttle/3200 RPM): 700 kph TAS
Top speed, WEP power: 751 kph TAS
With all tanks full you will lose about 4-5 kph or 3 knots.
When you do this testing make sure the airplane is stabilized and that all weather has been cleared to standard temperature and pressure so you have NO wind, NO icing, NO temperature, and NO atmospheric pressure variation from standard. That is done by clearing all weather. Then allow the plane to accelerate to top speed with NORMAL power (full throttle/3200 RPM but with MW50 turned OFF). You can use time compression if desired, 4x is about max without sucking too much fuel. Once the speed is completely stabilized, set the time compression back to 1x and engage MW50. This can be engaged from the VC by using the popup electrical panel in VC view, press Shift-6.
You should get the same speeds we got.