Do you have dual shaft motors? If so, making dampers may double your speeds.
I use 250 oz motors with a Xylotex, and 1/2-8 2 start screws. My dual driven X axis is set at 155ipm, and my Y axis is set at 190ipm.
But I had to add dampers to make is usable.
[nomedia="http://www.youtube.com/watch?v=UJjivsVwSsQ"]Stepper Motor Damper Test. - YouTube[/nomedia]