Hi Paul,
Actually, rather counter-intuitively, the lower the voltage rating the better in terms of speed. Long story short its because the windings have an inductance, and this means you want to run them at higher than rated voltage to get the best torque.
The main issue with the DA kit is the high gear ratio - the motor has to turn very quickly even at the sidereal rate. So going even faster with them causes the motors to stall - you can read the long explanation to find out why. Ideally the improvement is to reduce the gear ratio, but this is no simple task on the DA motors and because of their large step angle you get much lower resolution. I searched for ages to find a way to reuse the existing motor brackets and fit different motors/gearbox, but in the end decided it was simpler to just make new brackets from scratch.
Regards,
Tom.
Long Story:
As it is current that determines torque, then at high speeds, you need the current to reach its rated value as fast as possible in order to avoid stalling. As the motor steps faster and faster, the rise and fall times in the current become a larger portion of the step time, and hence on average torque goes down.
If however, you run a stepper motor at much higher than its rated voltage, and because for an inductor current rise time is proportional to voltage (V=L di/dt), then applying a larger voltage to a stepper motor will allow the coil to its rated current much faster, and hence the torque doesn't drop off as fast with speed.
This doesn't actually damage the motors as long as you limit the maximum current - if you drive a higher than rated voltage alone, the current will eventually reach a level higher than the motors rating causing it to get hot and if you go too far burn out. Again because the motors are inductive, you can use PWM to limit the current even whilst applying a higher voltage. In fact this is why the motors hiss when at low speed - the driver chips are PWMing to limit the current.
The reason motors with a lower voltage rating are usually better is because (1) you can overvoltage them on the 12V supply to get better speed as explained above, but also (2) they also tend to have less inductance - so the rise time is already less. The flip side of the coin is that lower voltage motors require higher currents for the same torque, and the drivers can only handle so much current (<1.5A max/phase), so that limits some of the very low voltage motors.
