To solve this we should need more exact figures e.g. from what state you started your flight. If it was from "Cold and Dark" it might well be that the flaps are still fully extended...

Just a hint, no offense intended. In fact we are not aware of any such discrepancies. Climb rate does of course vary with weight and external loads. Therefore the only available and reliable figures are derived from testflights with WEP - which is not recommended when not absolutely necessary. Nevertheless we of course have verified those during our Beta-Test. Still with a combat weight around 9700 lbs and a powersetting of 46"/2700 RPM you should get approximately 2000 FPM at 170 MPH IAS.