3500mm/min = 700 rpm, which about what you can expect.
According to Gecko, for maximum motor performance, the voltage should be 32 x sq root of the inductance. This is about 80V. Since you're running at 40V, you're only getting about 1/2 the maximum speed those motors are capable of.
So, you have 3 option.
1. Change the motors to lower inductance, higher current motors, that will have more torque at higher speeds.
2. Change the drives and power supply to run your motors at 80V.
3. Use a gear reduction to spin the screw faster.
I'd go with #3, as you suggested.
As you're currently set up, 25Khz kernel speed is good for about 4700mm/min
25000 steps/sec x 60 sec divided by 320steps/mm
Yes, but keep in mind that moving the machine twice as fast at any given motor speed requires twice as much torque. You'll definitely get more speed, but it may not be double.If I keep the base motor speed but increase shaft speed by a 2:1 pulley then I only get a 50% loss of torque for the same speed. Does that sound right?
Here's a good explanation of how spinning a stepper slower can give you higher speeds.
Mechanical Power - PMinMO.com