More information is required. What voltage and amperage battery are you using? What specific speed control and is it programmable? How hot are the motor and speed control getting?

On 6s you could easily go to an APC 16x8 E prop. You may have to set up a throttle curve to bring the power on sooner. I typically like to have cruise speed at half Stick which translates into 5/8-3/4 power at half Stick.

With an electric outrunner type motor that the OP has you are able to spin a larger prop at a slower rpm which we are finding to be more efficient for an airplane like the Kadet Sr. Pretty much the same reason why guys started flying 4 strokes on them only better. Another advantage of electric is that you don't need to prop to run in the power band. Electrics have torque at any rpm and as long as you don't exceed amperage for any of the components you can keep adding more prop. Depending on the type of airplane you can adjust cell count and prop size to suit the airplane type. Fewer cells allow for bigger props at less rpm for a Kadet type or more cells with a smaller prop at higher rpm for more sporty airplanes. Running a glow prop on an electric only reduces efficiency.