What I see is vignette at the short end that is fairly significant wide open. It isn't uncommon in still zooms. I don't know which others are better in this regard. Does anyone? I'm curious now to k ow how it compares to others. Sure one might expect better from an L series lens, but I can deal with it myself. Heck I'm so used to it that it didn't even occur to me when I first read the OP.
It appears more pronounced when the iris is open. Would that indicate that it is related to the curvature of the elements?