How can I speed up travel on this Boxford router? What new stepper motors to get?

[JAZZCNC]

Does your idea work because the drive is a constant current drive, which needs to supply a higher voltage the faster the motor turns, because the motor impeadace increases with frequency, so at some point the supply to to motor will reach the supply voltage of the driver and therefore not be able to get any higher. So increasing the driver supply voltage will give more headroom on upper frequency limit?

Yes you got it pretty much spot on.! . . So you can see the motor windings makes big difference and motor with high inductance will require more volts to reach same speed as motor with low inductance. This is why it's not advisable to use large motors when not required. Often people think bigger is better but in CNC this is wrong and often cripples the machine.

However in your case you need to be careful increasing voltage with your old drives because while the drive max spec is 46Vdc you'll need to leave decent overhead to allow for EMF. Also they are Old tech analog drives and IME not very strong.
Modern digital drives work so much better than Analog drives and handle motor resonance so much better that there really is no comparison. If you had the same voltage and motors just changing to digital drives makes enough of an improvement to be worth the cost. The difference is night and day in some cases.

Edit: Also just to make you aware the pulse quality and rate makes big difference to speed you'll get from machine. For the full affect in performance you'll need to change the controller to one that allows running at higher frequency and better quality than parallel port allows. However without hacking the old controller you'll need to do this anyway I think.
The machine I quoted had the full treatment and the combination of digital drives with higher frequency high quality controller and more volts is what allowed the speed increase. Also the machine run much smoother because of these upgrades.