[Alan Anderson]

Here's a simple option that might work well for you: in the wire between the joystick axis and the Robot Drive function, insert a cube function.

Right-click the wire. From the popup menu, choose Insert -> Numeric Palette -> Expression Node (it's a rectangle saying EXPR). You'll get a small orange box with right-pointing arrows on either end. Click the blank space between the arrows and type x*x*x.

Now, small motions of the joystick will give you very small changes in the motor control value. Medium motions of the joystick will give you smallish changes in the motor value. But full travel of the joystick will still give you full power.