thanks for the link Richard.
Mongo has the 35mm f1.4 Sigma ART and swears by the "Art" series. He much prefers his Sigma 35mm f1.4 to the Nikon equivalent and there are a few factors apart from sheer performance that come into play in that case; not the least of which is that it is less than half the price (and in Mongo's opinion, also outperforms the Nikon). It is not surprising that this new 85mm Sigma performs to the "ART" standard. It also seems that it very very closely performs as the Nikon does more so than with the Cannon.
However, as supportive as Mongo is of this Sigma series, he has to be objective. In the case of the 85mm Sigma, just on size/weight alone, Mongo would stick with the Nikon. The $400 difference is not significant enough to be a real consideration. If the lens were just sub $1000, it may come into play possibly with a lot of potential buyers.