I know some other heatsinks block off the sides, but I can only remember 1 off the top of my head: the Noctua U12P. And I mostly remember that because I have one. Even then, it doesn't completely
block off the sides like the Swiftech, only blocking about the middle half of it (i.e. 25%->75%).
This can be seen in the pics on the SPCR review for it:http://www.silentpcreview.com/article850-page2.html
Looking more at the pics of the Swiftech, it seems because of the curves on the front sides, this trait even blocks some of the air from leaving forwards. I wonder if it would do better without the front curves blocking air going out, because I'd imagine it then sends that air back toward the middle of the width heatsink, forcing the air to compete for what gets out.