A suggestion (from a non-plasma expert!) is to consider running a simple bearing from the back of the Z axis plate onto the top and bottom of the upper 40mm rail. This would give a bit more support to reduce the sag
While this seems to make good sense it would appear to me to transmit any imperfections in the ali section to the rail and carriage. If you go down this route then there is no reason to use the precision rail and and I would just mount more bearings and run it on the Ali sections full stop.

Barry, I see that is how you have done it on the x axis on your machine, do you get any problems with surface Finnish or the bearings compacting the Ali and causing a groove?