Well, you don't ever want to exceed the Red Line on your Airspeed indicator, and if my memory serves me correctly, that is about 199 KIAS for the Turbine Duke. Try taking it up to anywhere between 20,000 and 26,500 and put your throttles in the green as well as your prop RPM in the green. There, you should get maybe 190 KIAS but over 250 GS, given the right conditions.