Maximum speed of stepper motor
At 12 V the speed will soon be zero since something will overheat and break.  That's if this A4988 thing can even supply the necessary 7.5 A per phase.  If not, then it will probably get hot and break.  Either way, this is not a good idea.

There is one exception to this, which is if the 12 V is only applied for short periods of time to overcome the inductance of the windings, with the voltage then quickly brought back down to spec before the current exceeds spec.  That sort of drive can be useful for steppers because the current in the coils switches faster, which allows the motor to run faster.  However, care must be taken to not exceed the rated current.  Unless this A4988 thing is specifically designed to do this and you can set a current limit at the 1.78 A maximum the motor is rated at, the points in the first paragraph apply.