Can't get enough steps or maybe power

Hi All,

I have a new z-axis made from a TR18 leadscrew through a 1:16 gearbox, I've attached a Nema23 at first through the pololu driver and then through a 4A Pibot electronics stepper driver (TB6600) but it only functions up to around 5200 steps/unit. I'm using Marlin/Ramps1.3. Is there anyway of getting a higher step count (12800 would be great!) using my current hardware?

Thanks for any help.
/Trig.