Here is another quick test on all three lenses at f/2.8 where the difference is so minute that anyone could say is so irrelevant when it comes to photography. You could shoot these lenses and capture amazing photographs without any limitation of sharpness or color rendition. At this level of quality is up to you to make a great image. Some say that using other lenses on a Leica body turns out to lessen the quality of said lens, I do not know if this only qualifies when shooting JPEG only versus shooting in RAW.
I have been shooting the my Nikkor 85mm AF-D for about 3 years, I can say that is sharp from f/2.8 and on but if you shoot at f1.4 or f/2 get ready to see soft images and aberrations. The only lens I have shot below f/2.5 that can truly say is sharp is that Summilux-M 50 Asph. Keep in mind I have not shot the Zeiss 50 f/1.4 to compare but I have shot the Sigma 35mm Art and even that lens is sharp but compared to a Lux-M Asph at f/1.4 is still soft.
At the end of the day, the only time you should shoot at f/2 or bellow is if your subjects aren’t running otherwise good luck catching them in motion at those speeds, your percentage of good sharp photos will be less than 10% with the exeption of shooting from about 8 meters, then you will have good sharp images.