Interesting features, never looked at the Castle flying stuff much. Do you have the Castle Link? Check out the Governor Low and Governor High settings on the throttle tab, appears to do exactly what you want!
Phoenix Ice - Heli Programming Screenshot
Snip from the beginning of the last page of the online manual:
Governor mode acts as an RPM control, rather than a throttle control. Throttle stick position (or throttle curve %) determines the RPM that the motor will run and the controller will attempt to hold that RPM regardless of how the load (collective) changes.
For maintaining the speed as load goes off and on, try adjusting the Governor Gain, I expect up. I assume this is for adjusting the Proportional gain of the servo feedback loop, the first step in tuning a servo drive. Don’t try too much at once, too much gain makes the system unstable, although they may not give you that much control.
I don’t see the typical Derivative or Integral (aka Reset) functions of a common PID control loop, but if you do run across those I might be able to help more.
High voltage, good move, that will help with power.