Goran,

That is technically correct... the inductance of the motor affects the rate at which its torque drops of with increasing revs. The Nm rating of the motor is when it is stalled, a condition that rarely exists. A stepper motors torque is relatively constant, but below the stall torque, at low speeds until a point called the corner speed. Above those revs the torque (and therefore the ability of the motor to accelerate the load) drops rapidly. The corner speed is dictated by the inductance and the voltage.

Without knowing anything about your mechanical arrangements it is impossible to say what torque and revs you need. It is possible your mechanics are such that the motor never gets above a certain revs and therefore a higher voltage is not needed. Therefore advising you on driver selection is near impossible. It is also possible that the motors you have selected, despite their high torque, are unable to run fast enough for your requirements - larger motors do not rev as high as smaller ones. In either case you need to tell us a lot more about the mechanical arrangements.

There is a spreadsheet and tutorial here that you may find useful.

The approach that uses a multiplier of 10 - 20 x the motor voltage rating is a very rough rule of thumb and doesnt really apply to chopper based bipolar drivers but to the earlier unipolar L/R drivers. Like many 'rules of thumb' it is often quoted with no understanding of why (or if) it works. I would ignore it in favour of the other approach.