Any recommendations for 20mm or 21mm manual prime?

A big problem with ZM lens is corner smearing. I really don't see a point of buying such an expensive and reasonably fast lens and be able to only use it heavily stopped down. Voigtländer f/4 lens look a bit better value for the money if they are reasonably sharp at f/5.6 and very good at f/8. I would even consider 25mm if it's sharper than 21mm on X-E1. Lens size and thumb focusing looks compelling too.