If you have problems with throttle range, the easy solution is to wait a few seconds until the esc beeps after plugging in the battery. Don't move the throttle until it beeps
I just tried another test using a Zippy 3 cell 2200 25C battery and did as you suggested. Again, RPMs increased to maybe 70% of throttle range. No increase in RPM from there to WOT. However, judging from how hard the plane is to hold, the thrust may be at max for the motor at the 70& position. Is that possible?