Mike,
Didn't see a motor curve for the 425's but did find one for the 156's you currently have. Without a motor curve at the supplied voltage, amps, and pulse rate one would use we would be only guessing the preformance to be had by a motor unless you had a very similar system to compare to. So i am going to ramble some in general.
Using a G201, micro stepping at 10, 2:1 ratio, 8 tpi screw gives you the 32000 steps per inch. At 50 ipm your current motor
rpm would be 800. At that rpm you are on the bottom of the motor curve in terms of torque. At 50 IPM you have about 55 in oz
available and at 70 ipm you skip because the torque drops to maybe 40 - 45 in oz. That suggests that you need 50 in oz to move
the axis and it leaves no power for acceleration and machining. Make note that the motor curve is for 30V & half step, But irrelevent your motors are just turning fast with no torque. 40 oz in can be achieved with two fingers and light pressure to turn something.
Should you change out the screws to multi start you can reduce the rpm and move the motors operating rpm into a higer torque region. 2 start screws would almost / maybe double the torque available. The calculated resolution is still good but you'll never achieve it with the screws you currently have. Would guess that you could live with .001" resolution with wood working and probably even less.
If you look at the 382 in oz you'll find a curve but that curve is for a differnet motor 500 in oz and Keling can't confirm what it's for. You can still look at the curve to see it's affect on incresaing the torque and feedrate. Never found anything which provides power required to do wood working.
There are router users in here and probably someone will be additive to this post.
RICH