marlin firmware and half height prints

[printbus]

I agree with Roxy. For comparison, here are the numbers MakerFarm uses as their values, and their printers can likely be assumed to be close to what you have.

#define DEFAULT_MAX_FEEDRATE {250, 250, 2, 22} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {1000,1000,5,1000} // X, Y, Z, E maximum start speed for accelerated moves.

These are considerably slower than what appear to be Marlin defaults that you're using now.

On the Pololu driver boards, I assume you're referring to Pololu Black as the higher current driver board? I think the only difference is a more efficient heat dissipation scheme on the board. IMO, unless you've noticed the heatsink on the driver board running hot (well, I'm assuming nwreprap has you put heatsinks on the driver chips), buying the fancier driver board won't get you much. You can always set the Z-motor driver for 2x the setting of the drivers with one motor if you haven't already.

If you go to the Acme screws, remember you'll have to go back in and readjust the Z value in the DEFAULT_STEPS_PER_UNIT since they have a significantly different thread.

Have you been in touch with nwreprap at all over this, since they say the firmware files they provide are pretailored for you?