Hi Mike,
whether you use ordinary steppers, closed loop steppers or servos is not that important, any of these devices can do the job.
If ger21 is correct that your lead screws are 1/4 diameter then the speed at which you can rotate them before resonance is limited,
additionally they will flex torsionally for a loss of accuracy and if you up the torque the problem gets worse.
Look at your machine and see if you cant replace those lead screws with, ideally ground ball screws (really really expensive),or
rolled screws. Ebay has zillions of them, go for 16mm ones if you can get them to fit. They typically have a pitch of 4,5 or 6mm
so to spin them at 1000 rpm means rapids of 4m, 5m or 6m per minute, very useful.
If it proves impossible to replace your existing lead screws then significantly increasing the speed of your machine will be limited.
Under those circumstances better specified but otherwise standard steppers with decent drivers and a 68V+ power supply would easily
provide an increase in speed up to the resonance limit of the machine whilst conserving you budget. You could certainly achieve similar
results with servos say, they would cost a lot more and you will still only be able to turn up the wick to the limit imposed by the
mechanics of your machine not the capability of the servos.
Craig