I see no need to change ESCs when these work great. Also, not sure why you say the Focbox is better...... The peak amps on that controller is 120. I have pulled over 300 amps from Castle ESCs reliably.......
The sprocket size is based on the fact that the board uses a two stage reduction, belt drive 5 to 1 reduction from the motor to the jackshaft and chain drive 3.2 to 1 from the jackshaft to the axle. That gives a ratio of 16 to 1 from the motor to the axle.
1% of the world's population can think "Outside the box". The rest are firmly stuck within the box. Where are you?