As an astrophotographer I produced a short vignetting and corner sharpness comparison (1.4-2.
between the new Sigma 85/1.4 Art (S) with the Canon 85/1.2 L II (C). In short the S has maybe a little bit more vignetting BUT a much much better corner-sharpness (all pics are uncorrected and unsharpened and exposed to the same level (e.g. 1.4/4s/ISO800) under urban sky to show the vignetting! camera was a eos 5d III)