I have used the M-mount 21/4 for years on the M8.
My lens is not coded, and it produces pretty heavy color shifting and it also vignettes significantly. Cornerfix or an equivalent solution takes care of these issues. Coding the lens may give the same results with less post-processing hassle, but I haven't seen a direct comparison to confirm.
The lens is a joy to use. It is very small and handles very well. The optical quality is very good, no problems on M8 apart from the color and vignetting correction. The vignetting correction may bring about noise issues in the corners in low light/high ISO, but I have not specifically tested this myself.
Any improvement over this lens comes with a hefty price tag.