There isn't 'much' difference between the 85L II and the Sigma 85 but from 1.2-2.8 the Canon is sharper in the center - but we are talking about small amounts.
If you go to SLRgear.com and look at their blurr charts on full frame for both cameras you can see sharpness across the frame at major apertures.
Finally, there is no conceivable way, remotely, that a Sigma at 1.4 beats the Canon at 2.8 - not a chance in hell unless you have a damaged lens.
If you remove the Canon's F1.2 to F1.6 CA, something that can mostly be corrected in PP, the Canon is one of the Sharpest lenses ever produced stopped down to F4 - sharper than any lens I own - and I own many many L lenses. I have actually test both the new 35L II and the 85L II at F4 and IMO the 85L II is sharper and allows more fine detail to be captured using a 5DSR
Go look at the slrgear.com blurr chart and you will see it.
I have captured pictures that are used to reproduce large format oil paintings, to make prints for the artist, and I choose the Canon 85L II as the lens I use to do so - on a 5DSR, you can capture any amount of detail you want.
Thanks for the link!