800 would be correct if your electronics is set for 32 microsteps per step. If your electronics is set for the more common 16 microsteps per step, 400 would be the correct value.

This is determined from the "lead" spec on your Acme rods. The value of 8mm means the screw advances 8mm per revolution of the rod. Typical stepper motors use 200 full-size steps per revolution. For level 16 microstepping, this is 200*16 or 3200 microsteps per rev. For level 32 microstepping, this is 200*32 or 6400 microsteps per rev . Taking the 32 microstepping as an example, 3200 microsteps per rev on the motor * 1 rev per 8mm travel on the rods gives 800 microsteps per mm of travel.

Is the z-axis also slow to move when just doing a z-axis move and not a three-axis autohome? I'd focus on that. I don't have acme screws but I believe they can require a lot of torque to get the carriage moving. You probably want the stepper driver set for as high of a current as you can. Start with low values for z-axis acceleration and feedrate. Work your way up until you figure out what your new limits are. You might find you have to stick to a low acceleration factor on the z-axis.

There have been a few that have converted to acme screws. Maybe one of them will catch your thread and chime in on any other details.