Hi Guil
With a non-governing ESC and a FM9, use the throttle compensating mode. What this does is adds throttle over the duration of the flight to maintain the motor RPM and compensate for the deduction in voltage as the battery discharges. I do not have my FM9 Programmer handy but there are something like 16 settings for the style of compensation, use the highest (#16) for your setting. Then as John stated you will have to take a few tries to get the RPM you want. I suggest starting at about 70% throttle then change to suit.
In the throttle compensation mode you will probably want to fly about 100-200 RPM faster in level flight than you do with a Castle Phoenix. Without governing the airspeed will slow some in consecutive maneuvers, but it will still outperform the typical glow engine. It will also be a little more prone to wind-up in the wind than a governed system, but again, better (less wind-up) than a glow engine.