400 Hz translates to 24000 RPM, which is quite high, and should be fine for vast majority of motors.
Remember that this is your sampling frequency, not the motor speed. You would probably want to sample at least a hundred times per revolution in order to get a reasonable angular resolution. Given that the effects of imbalance are proportional to speed squared you really need to be spinning the motor somewhere near its normal working speed to balance it. If that's 10,000 rpm, it would need a sampling frequency of 16600 Hz to give 100 samples per revolution. Maybe you don't need that many samples per revolution - I don't know how many you need in practice, but I'm sure it's a lot more than 1.