Why do you want to change to a different motor?
Is this rack and pinion? If so, what size is the pinion?
What drives are you using, and what voltage are you running at?
What are the specs of the motors? (Both of them)
How much weight are you moving?
I would not expect a 670oz Nema 23 to be faster. Keep in mind the torque rating of a stepper is the torque when it's not spinning. I would expect the Nema 23 would have less torque at the speeds you need.
All of the things I mentioned have an affect on speed.