3 Axis CNC router

[JAZZCNC]

Would the larger mains voltage driver make a difference to speed?

Well yes it would but still you'll be limited mostly by the 5mm pitch. Simple maths really 1000rpm would give 5mtr/min but with 10mm that would be 10mtr/min for same RPM. The larger motors start to lose Torque quickly after 1000rpm so at best you'll get 1500rpm which is only 7500mtr/min.
Even then at 1500rpm your in the region where screw whip can start to become issue.

Are you saying a 4nm 23 could drive both of the screws if they were 2010? If so that would be a significant saving...

Yes it would, esp if 16mm dia but like I say you'll need ratio to increase torque. This will come at the cost of speed so you'll still be down in the 7500mtr/min area.

If it was me I'd go with 34 and run them on 70vdc and see what you get. 8nm should get you around 5-6mtr/min before saturation takes affect. Then if want little extra speed apply ratio to increase. 8nm will have enough spare power to handle the ratio.