Sherline (and Taig) used 20 TPI screws to get all the drive reduction needed in one step. The Kerk screws are really good screws. The combination of size and pitch used makes for a much more efficient screw but you are still loosing all that mechanical advantage (reduction) from the higher pitch stock screw.
It is very common to see commercial machines use a belt reduction on both stepper driven and servo driven machines. In our DIY CNC community some folks are opposed to the perceived added complication (and cost) of adding a belt reduction to each axis and try to compensate by adding huge motors.
Taking the Sherline as an example the stock 20 TPI screw will give you 20 times the torque of the stepper: 20 * 135 = 2700 oz-in. With the same motors and a 4 TPI screw you have: 540 oz-in. (I using holding torque numbers here for simplicity). So through the entire operating range you have less torque available to push the stock past the bit. That is why you said you had to take light cuts.