When I was trying to narrow down a problem one of the things I did was rewire so every motor went to it’s own stepper driver. The problem ended up not actually being related to the wiring, but since I had done all the running of the wire already anyway I went ahead and populated the RAMPs board with 2 extra stepper drivers and broke out every motor onto it’s own driver.
From what I’ve seen, I can go up to .8V and still have a stepper that is only at 60C. If I go up to .9V the stepper goes up to 70C which is too close to the 80C I have read that stepper motors start to lose magnetism at. I’ve ordered some 40mm x 40mm heatsinks I’m going to put on my Z axis and see if it can do .9V and still stay around 60C. For actual milling I’ve always found the Z axis having the proper torque is more important than any other motor. If the Z axis gets pulled down into the wood (like say if your bit becomes a bit dull in the middle of a large cut), things go very bad very quickly where if the X or Y axis lose steps it messes up your work but at least it doesn’t get pulled down into your spoilboard.
I know at .8V if I really try – I can move the Z axis while it’s sitting idle (holding torque), but at .9V I can’t physically do it with my hands – so there is definitely a noticeable strength difference even as high as .8 to .9V. Cooling the drivers is very important – I personally use a fan I pulled out of an enterprise level network switch – loud as can be, the noise coming from it could be heard through the rack cabinet, and through a wall – but boy does it push some air. When you’re milling you already have a ton of noise (granted the DWP611 is MUCH quieter than my DW660 ever was) so a loud fan doesn’t matter.