I've used many different 21's, including the SA f/3.4 and f/4, 21/2.8 non-asph and ASPH, Ricoh f/3.5, CV f/4, Zeiss f/2.8 and new and old f/4.5's, WATE. Not all on the M8, but most including the non-asph and ASPH.
The non-asph doesn't come off that well in comparison, but better on the M8 than on film. On film it's probably one of the poorest of the lot; not really much better than the SA f/4. But in any case, the new ASPH is the best of the f/2.8's, and the Zeiss is not far behind. The new f/4.5 Zeiss is probably the best overall, but not much better than the ASPH. The WATE is outstanding for a zoom and in my view about the same as the Zeiss f/2.8.
I still have the SA f/3.4 for film, the CV for compactness and the WATE for convenience. I use the ASPH when I'm sure I'm going to need the quality or speed.
Henning