The voltage will determine the speed that the steppers will still have enough torque to move the axes, which in essence determines the maximum rapids of the machine. Or in other words, the torque won't drop off as fast with higher voltage.
The other thing is your federate is determined by the spindle speed; in the case of a typical mill around 4000-5000rpm. The electric spindle the OP is using is capable of 5-6 times that, with the power band of that spindle toward the upper region of its RPM range. This means the machine should be running at a faster federate, likely with less depth of cut, than your mill.
We don't know exactly what type of machining the OP intends to do with his machine, but a lot of what the machine will do will be determined by the components he purchases and not the other way around.
Obviously, in your case, you could have bought 380oz-in steppers and ran them at the max current setting of the drive, had better acceleration, and saved a few bucks over purchasing the 570oz-in steppers. But this is why I always stress to prospective builders to make these estimates of what's needed BEFORE beginning a build, or at the very least size the electronics depending on what you need from your machine instead of designing a machine around your steppers.