One easy way to up the speed of your z-axis is to use geared belts to change the ratio that drives the lead screw of your axis.
Check out my setup here, which also has links to the parts I bought.
If you choose to go the route of changing the motor, you may run into some issues with finding a compatible encoder. There are threads that address this issue, so it is certainly not insurmountable, but from an ease of implementation standpoint, the gears worked great for me.