I reached out to Mike with the same question about the gear ratio and he said it can be changed by replacing the pinion gear. He supplies several different types so you can speed up or slow down. The way they are setup is the stock Heng Long motor will turn his grearboxes the same rate as stock Heng Long/Taigen gears. Change the supplied pinions and you change the ratio.
Hi you can slow down the 540 versions by putting crawler motors in also gives you more torque. But I wouldn't advise running 540 motors on a standard board. if you want more torque on the other gearboxes try 480 motors.