I used Routout 2.5 amps drives on my machine to drive 220Ncm steppers. These were 8 wire motors rated at 2.5amps at 7.5 volts
Originally I wired them in series, and, like you, I found them lacking - only moving my lathe at 4 inches pm (reliably). (Mine is geared down at 3 to 1)
I wanted then to "rewire" the motors in parrallel. The problem was, of course, when I checked the windings and did the sums, the rating was for a single coil, and as soon as I put the coils in parrallel, I neede more omph than the Routout could provide.
To be fair, the Routouts did run the parrallel set up, better than when the motors were in series, but they were limited to 2.5amps and 24 volts.
I got a Gecko from a friend (and have since bought another) - and these are in a different league. They are adjustable, and rated up to 7 amps, and up to 80 volts. Fastening these on (using the same motors at 24 volts - same as Routout) my speeds went up to 40 ins per minute. I admit they are not reliable at that speed and I have dropped them down to 20 ins pm but a five fold increase is great. I am still using the Rotout card to run my milling head - but when I have the cash to invest in a third Gecko, I will then try a higher voltage - maybe 36 volts, and see what improvement that brings.
Before you rush to change motors, I would honestly fit yourself up with 3 good drives - the Gecko 201's are ideal. The beauty about that is you have the capacity to change current and voltage to your motors, without have to change your drives again. You can then see if you can coax any better performance from your motors.