The Leica 180mm f/2.8 APO-Elmarit-R has a cult following among Nikon and Canon users (converted for Nikon or with an adapter for Canon). I bought mine some years ago brand-new (latest variant), back when it was still in production.
Given the stellar reputation of the Leica 180mm, can one reasonably expect that a Sigma lens might compete with it? Using the expression “when pigs fly” (foreign readers might find that one odd), well, pigs are airborne as we speak.
The comparison is also cross-posted in Guide to Leica.