Although my threaded rod was rough and my lead screw is smooth, I think adding the second threaded rod on z axis made the biggest difference to my print quality - really reduced the ripples due to bed lateral vibration during fast direction changes (infill). See Mandrav's build or my build log for pictures. Cost of a lead screw with nut is under £10 I think. You could buy three for £15 when I last looked, if you can wait for mail from China. I imagine if you are with the Smartcore community, you're probably an expert in waiting for parts from China! Don't forget you need to adapt/redesign your printed parts. I got away with adapting the one I had for a single screw but decided to completely redesign the z bracket for my 2 stepper system. Files are shared in my build log and I think they're on Thingiverse if you look for my same username. Main benefit for me was spacing the smooth rods further apart for more lateral rigidity. Obviously a second z stepper really benefits from a second pair of smooth rods (more cost) but I wouldn't rule out seeing some improvements from just having the second lead screw. Perhaps not worth the extra cost of lead screw and stepper if you're not going to add the extra rods though. Rambling now, so will off to bed.
