The Babymaker can easily achieve 20 MPH. For legal reasons, if it reaches speeds above 28 MPH (15 KPH) the motor cuts out and it’s up to the rider how fast they can pedal. The bikes all have a speedometer on the display, so you can see exactly how fast that is.
The Babymaker is equipped with a 350W motor. The motor's peak watt output is roughly 600w. To prevent overheating, peak output will typically only occur from a standstill for a quick boost or when climbing hills. Nominal voltage can be achieved at any given speed. That's where variables such as rider weight, strength, weather conditions, and road conditions come into play. As an example; if the nominal voltage is achieved at 20mph based on your scenario, then that's the max amount of assist you will get from the bike.