It depends on what you set the step multiplier to. If set to 1, you need 400 inches divided by 60seconds, times 5 tpi, times 2000 steps per turn, which is 66,666 Khz. Set the multiplier to 2 and you'd only need half that. I think this is right.
No, not the voltage. The software controller (Mach2, for example) has settings for acceleration, and sends that information to the Gecko's based on the programmed feed rates and distances moved.