OK, I installed the 40 tooth lead screw pulleys and anti-backlash nuts last night.
I am still using the regular 4-lead (8mm pitch) lead screws.
This results in a resolution of 0.02mm per full step or 0.00125mm per microstep (for reference only).
I changed the firmware settings to 800 steps per mm for the Z axis of course, but changed nothing else.
I put a thin smear of THK grease on the lead screws as I have done previously.
1. The Z motion is both smoother and quieter
2. A test print with 0.2mm layers turned out considerably nicer than I am accustomed to with very consistent layers
3. An ABS test print using 0.15mm layers turned surprisingly nicely and required almost no sanding and vapor polishing to become positively glossy (better than normal)
To put that into perspective, I have never had a great deal of luck using layers thinner than 0.2mm in the past.
Now I feel confident that I can probably run 0.1mm layers or even smaller and may try them later today or tomorrow.
So these appear to be a definite improvement as far as I am concerned.
How much of that is due to the new 2:1 pulley reduction ratio or to the anti-backlash nuts is a matter for conjecture, but regardless it is noticeably better.
I did a 0.1mm layer height print over the weekend and it turned out excellent.
I may try going even lower in layer height.
I am not sure what the practical limit will be, but it was a tiny part (a connector clip) and the layers were as even as could be.