I just thought of something else. With 3 mm filament, variances in the diameter of the filament have a much larger effect on the actual amount of plastic extruded. We've seen the diameter in different places on the same spool vary but up to 0.10 mm. This means that you could be pushing out 10% more filament (by volume) in some spots than in other spots. I wonder how much of the Z banding we see is actually caused by differences in the filament diameter.