Actually that's not the case oppie. The head speed is slower, not faster, with the 9T motor. I assume Horizon does this since a 3S lipo will speed the motor up, and they want to run the same head speed. Now, an 11 tooth motor will turn the blades faster, but a 9t is slower, notwithstanding what higher/ lower voltages do to even things out.

To the original poster: did you correctly set the gear mesh? There is most certainly enough slop in the screw holes to set up the mesh too loose or too tight.