By going with a higher-voltage battery, you will increase the Amps. The 5 Amp ESC is borderline on the stock Slow Stick, so the 9.6V battery will probably push it over the edge.

Amps increase or decrease in proportion to both the load (prop/gearbox/motor) and the Volts: Keep the load the same, but increase Volts, and Amps increase. Keep the Volts the same, but increase load, and Amps increase. Increase Volts, and decrease load, and Amps stay the same.