Hey there. I started to look at it.. and it got pretty clear right off. http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=113&Camera=453&Sample=0&FLI=0&API= 2&LensComp=374&CameraComp=9&SampleComp=0&FLIComp=1 &APIComp=2

Those are Bryan's ISO crops for sharpness. Great tool. Doesn't look like he's done a review on the lens, but he has at least done that much.

Personally, I'm not familiar with the Sigma. Is it in the same price range as the 100-400? You'll find a bunch of advocates of that lens on this site.