I'm quite happy with my new sigma 50/1.4 art now replacing the canon 50/1.2 in my collection. Use it on my 5D3 and 5D2 (astromod) bodies.
Wanna share my quick and personal test for the best aperture of my new Sigma 50/1.4 ART on my astro-modified canon eos 5D3. The pics are showing stripes (centre and below extreme corner lower right) with apertures from 5.6, 4, 2.8, 2, 1.4 with equivalent exposures between 160-10s. raw developed in DXO with vignetting and distortion switched off! My personal conclusion: vignetting invisible at 2.8, perfect sharpness and aberrations at 4.0 but already very good and quite usable for astro work at 2.8 - this is two stops better than my (now sold) 50/1.2 copy...
(the strange color comes from the UHC clip in filter and the high background)
pic 1: centre (5.6-1.4) , 100%
pic 2: corner (5.6-1.4) , 100%