I have the 35mm ART and compared it to my Nikon 24-70mm f/2.8G and it came out a little softer than the Nikon using the latest Reikan FoCal software.
My ART seems to have a shift of the AF in the Red, Blue, and Green when I did the FoCal AF test on it. The Red needs a -17, The Blue a -15, and the Green -10 in the body for AF tuning.
The Nikon sets around -14 with a two point spread in the RGB, vs. the Sigma 7 point spread above.
However, at least you can calibrate the lens AF tuning with their puck to a given body, just not the aberrations of the three colors though.