Interesting you noticed the CA.
I have the 35mm ART and I used their tuning puck to fine tune the AF which helped as it tunes over four different distance ranges.
However, I also have the latest beta of Reikan FoCal and I let it run though the test on a Nikon D800E and compared it against the 24-70mm f/2.8 Nikon. The Nikon had far better CA results than the Sigma which surprised me for a zoom. The beta software will draw out the Red, Blue, and Green "best focus tuning curves" during the testing and the Sigma was pretty far apart against the Nikon's curves so the unsharpness may be the result of the CA matter with my 35mm ART. Sort of interesting, but I like the looks and size of the Sigma over the Nikon (It's better looking, imho.) and it is faster at f/1.4 so I'll keep it for when I need a fast prime. Not a big deal breaker, but it's got some issues.
Fwiw, I also own the first iteration of the Tokina 11-16mm f/2.8 DX lens and it was really bad for CA. They came out with the II version shortly after that to address some of it. In FoCal, the software has issues even trying to give me a decent AF Tuning number it is so bad. My first FoCal beta came out with "You need to set your camera's AF to -45" which is impossible on a Nikon as it only goes to -20. Go figger.
SG