I think that's working out at around 460rpm atm at the stepper at 90ipm.
I would go with an AC servo 750w-1kw that has 1500-3000rpm.

I get 600rpm on my mill using 60V to run my nema23's (566oz).

If you belted it 2:1 you would likely half the existing torque (1 rev of stepper = 2 rev of screw).