Changing the pitch of your ballscrews is the same as selecting a 2:1 overdrive for your motor, so you will be cutting your effective motor torque in half.
I honestly don't know about stepper drives that much, but it seems to me that they are pretty cut and dried: the driver puts out a pulse and the motor had better follow it, or else you lose a step and your position is out the window.
A servo motor, whether Brushless AC or a conventional DC motor are the industry standard in reliability. The setup for servo motors has many configuration parameters that allow you to tune the motor to your system load and desired response. Even if the servos cost you a bit more, you are getting more. I think the drives are more nearly equal in price, so you'd only have to buy those once anyways
When running a high feedrate, you also need to be concerned with servo lag, just so you know. The motors respond quickly, but the higher your feedrate, the more time it takes them to accel/decel, and this is why special software is required to run these types of machines, because the software reads ahead in the code and anticipates sharp turns and begins to modify the feedrate early on, so that the motor does not overshoot (and possibly gouge) the corners.
I'd encourage you to look at CamsoftCorp software for your CNC controller. It has provisions (in the CNC Professional package) to help you control high feedrates. I have never used that part of the package, but I know its in there
The maximum speed of a DC servo is proportional to the voltage applied, so keep that in mind when you pick out power supplies. The rating of the motor does not guarantee the nameplate torque if you are low on available driver voltage.