Quick update after quick test sigma 24mm. My copy has the following characteristics:
1. It hunts more/focuses (much) slower than the 35mm art lens in low light.
2. suffers from front-focusing
3. heavy vignetting up to f/4 (but 35mm has that as well)
4. sharpness comparable to (or a bit less than) 35mm art lens.
5. and, most surprisingly, no (let me be carefully: hardly any) CA.
6. pleasing (or acceptable, depending on your preference) color and bokeh rendering although the "bubbles" are not perfectly round.
7. did not check distortion.
I am still hesitating if I should trade it for one of the Nikon 24mm's because of the focusing. But the lack of CA may offset that disadvantage.