It depends entirely on what sort of motor you have as to the maximum speed you can obtain.
have tried a stepper nema 23 with 2Amp per phase and 12V
NEMA 23 just tells you about the physical size of the end plate of the motor, it has nothing to do with how much torque it can delver nor what speed it can go at. So we have no idea if you are under running or over running the motor by giving it 2A.