Consider that on a good day under ideal conditions, the stepper motor is going to max out at about 900 RPM reliably. This is because the torque drops off rapidly with an increase in RPM. Above that, the stepper may not even have enough torque to turn itself, let alone do any useful work.

Your item #3 above. Yes, that is correct. If you set the motor for 1/2 stepping, that will give you 400 steps per revolution. This is normally the best performance setting, but try some other settings to see if you get better performance.