Jeff, I have a similarly sized machine (25"x32", 8020 aluminum), with similar motors. 5 TPI ballscrews, linear rails - very smooth and fast (250 IPM) with my 9.6V cordless.
However, with my 425 oz/in NEMA 23 steppers I hit a wall with my rapids around 100 IPM. I can cut very reliably at 90 IPM, which is ok, but I can't do much safely above that.
One theory is that I have a slow computer (800Mhz), and that Mach is not running as fast as it should, or that the timing might be off. I have a 1.6GHz machine I need to test this theory against, but I believe this is not the problem.
Second theory is that this particular motor is only effective until about 500 RPM (100 IPM for me). I've seen references to this in several places and the torque charts seem to bear this out. The torque is about 50% at 500 RPM and drops to about 17% at 1000 RPM. Some of the smaller steppers (like the 270 oz/in size) actually produce more torque at 500 and 1000 RPM than the larger 425 oz/in do. The smaller motor has a much flatter, more linear graph.
@ 500 RPM, 269 oz/in = 1.3 Nm, 425 oz/in = 1.1 Nm
@ 1000 RPM 269 oz/in = .9 Nm, 425 oz/in = .45 Nm
Right now, I'm really leaning towards theory #2, but it's hard to say. Initially, I thought the problem could be fixed with higher voltage. I was running a Xylotex 24VDC setup and then switched to Gecko G203Vs and a 72VDC, with no real change in performance. That was an expensive experiment. For me, it's not voltage/current/mid-band resonance - it's something else.
In your case, the 1/2-10 lead screws aren't doing you any favors, since you're going to need 1000 RPM to hit the 100 IPM mark and the motors may not have the juice to get you there. I'm skeptical that a Smoothstepper is going to help you out here, but I'm definitely no expert.
If my computer upgrade fails to get me anywhere, I'm considering swapping in a different motor(s), a different screw (2 TPI), trying a timing belt reducer, or if all else fails - maybe it's time for a Smoothstepper.
If I upgrade my X/Y to a NEMA 34 640 oz/in, I should have loads of torque:
@500 RPM, 640 oz/in = 3.6 Nm
@1000 RPM 640 oz/in = 2.3 Nm
This motor has a very flat, linear torque curve and much more available torque than what we're running now. I have to believe this is the proper solution. Again, there may be something else going on here, but this is what I am working on at the moment.
Please keep this thread updated with your progress and I'll do the same.
Steve - sorry for the long-winded response