Advice needed?.

Help!!!!.
I am in the process of building and designing a gantry type
mill/router.It macines approx 1500mm*1000mm*300mm.
I am using leadscrews (16mm dia * 4mm pitch),with phospher bronze
anti backlash nuts.These are driven from a 300oz stepper motor
through a timing belt on 1:1 pulleys.
I am currently only achieving 0.8"/sec max feed before the motor
stalls.
How can i improve this?.
I am looking at new motors which are 700oz.
If i detatch the belt the motors will actually run up to 3"/sec
before stalling,so would i achieve anything by gearing down maybe 2:1
to increase torque so increase speed.Or would ballscrews etc be the
best bet.
I would like to hear from anybody that has built or is building
something similar.

thanx
mark