I've spent some time trying to figure the z-ribbing out. There are some very good write-ups on printrbottalk about it. However, when folks finally started calculating the real values for the ribbing you can expect for having mis-matched screw pitch and step the values were in the single-digit micron range. This is sub-microscopic. The conclusion is that visible z-ribbing is a result of lead screw wobble, loose screws, etc. The lead screw measurements do not effect visible z-ribbing at all. Please feel free to head on over to their forum and do some reading on this.