This is a very new lens and finding a review was pretty difficult, but here's one that seems to describe exactly what's happening here.
http://www.slrgear.com/reviews/showp...uct/993/cat/13
Click on the graph on the R/H side of the page. It shows that the lens has 0.73EV of light falloff in the corners at 135mm when it's wide open at f/5.6.
It seems that this lens is incredibly sharp, but it has come at the expense of having high levels of distortion and vignetting.