If you go to 1/8 mico step the numbers are not much worse, 13280/sec
You are still well within Mach3's capabilities at its lowest settings.
I am assuming that your .1 screw is 10tpi.
4000 steps/inch would indicate that you are running in 1/2 step mode.
One of the underlying factors is that those motors are rated for a max voltage of around 80v if memory serves.
Running them near half of max makes for slow going.
going to a .2 pitch screw means that you will be able to go almost twice as fast but not quite due to the increased power demands of going faster.