When I contacted Astrosyn I was adviced for the best speed and torque to run the steppers at 30-40volts not exceeding 1amp per phase.
Yes, if you don't have sufficient voltage it'll severely limit the top speed

One alternative is to use resisters instead of the LM317s. Resisters wont give you a perfect linear current drive, but you may be able to increase the supply voltage to compensate and, since they'll run to a higher temperature before failure, the extra dissipation is less of an issue.

The real answer is to get yourself a better bipolar driver with switch mode current limit (pwm ideally).