10mm belt nema23

[paulus.v]

You can go with the voltage higher and you will want to do that for better torque at high speeds.
According to the rated inductance you can go up to 32*sqrt(inductance)=43 volt.
Or if you take the rated voltage you have a max. of 20*2.6V=52 volt.

First calc. is more accurate but you have +/-20% inductance accuracy.
Personally I would try with 45-46 volt, but I don't know how would the drives tolerate that voltage. Maybe there are others here with more experience in these drives.
Have you tried wirh the drives set to the maximum amps rating? (SW1, SW2 and SW3 off)

It is possible to be able to improve your speeds in other ways as well. How easy are your axes sliding on their guides? The quality of the belts and pulleys, the weight of the gantry, reducing the inertia of the rotational loads, etc