You are actually right chorner, the APC obtains higher rpm because of its slender profile near the tips. When I tried to fly the engine at a 35C day, the plane could barely make the first turn at full throttle, despite the fact that the peak rpm on the ground was good and the motor did not overheat (temp sensors). Means that the thinner profile does not pull well in thinner air. Since this is a 4-st with lots of torque, probably a wider chord blade at the tips would fix this kind of thrust deficit...