Now, 2/3 throttle brings me around 5.5 amps at around 11V or so.
Here is the question: does that mean my motor is running close to peak efficiency at 2/3 throttle?
We don't really know. We can calculate the torque needed for the prop RPM at 2/3 throttle. Then we know how much current is needed, and we know what the motor voltage is. but it's not that simple as you can easily lose 10% efficiency, that we can't really define easily.