The castle engineers we spoke with basically made the comment that it was calibrated against being reasonable near the upper end of the range, and that if you are pulling 3A or less (which is what we do) that it just won't be close to accurate anymore, particularly with the sampling rate that is used.
The autopilot definetly should be sent this information. With the information it becomes possible to improve a large number of failsafe conditions/sensor error detecting. (IE on plane knowing the power output on the motor, would let us correlate the airspeed reading with the throttle and evaluate if the airspeed sensor is blowing out). On a copter it would give you definitive reporting on which ESC is stopped/not spinning correctly, which allows you to handle mechanical failures with more confidence (IE loss of prop/bad power connection is now detectable, outside of just pushing the outputs to max)