@johno4321 said in Stepper motors & Z lead screws:

As I understand it changing to a 0.9 degree stepper will increase print precision but decrease maximum motor speed - is this correct?

yes it will reduce max speed. but you wont be operating at those speeds anyway.
for a discussion of 0.9 vs 1.8 on prusa printer have a look at this. https://forum.prusaprinters.org/forum/original-prusa-i3-mk3s-mk3-user-mods-octoprint-enclosures-nozzles-.../stepper-motor-upgrades-to-eliminate-vfa-s-vertical-fine-artifacts/

blv recommends 0.9 x and y degrees for his newer project, the blv qube.

you might also want to replace the couplers with jaw couplers, or place one ball bearing inside the coupler to reduce any springiness of those couplers.