they (SLRgear) tested Otus yesterday (posted) btw... minor note - Otus was provided by R.Cicala from Lens Rentals, so I bet that copy of Otus was the best one (in terms of decentering) from many, while Sigma was just a stock one.
Hum... considering that the Sigma is not for sales yet, I would think that the sample was cherry picked by Sigma.
I would be surprised if Lens Rentals had tens of copies of the Otus, so it is most probably just an average lens. But I am not sure this is even relevant since one of the key reasons why it costs what it costs is because manufacturing tolerances and quality control are done according to very high standards. I am yet to hear complains about decentring with the Otus.
In other words, I think that you probably have it backwards...
Don't get me wrong, I think the Sigma is an outstanding offering that is clearly putting Nikon and Canon to shame. Those guys need to do a very deep introspection... not only are they losing significant market share to mirror less entrants... but they get their ass kicked big time right in the middle of their own garden by a company much smaller than they are.
Now, at least Nikon has made it very clear that their top priority these days is lens rendering... even though pretty much nobody understands this outside of Japan... good for Sigma and the Fukushima economy!