FYI I finally took the plunge & sprung for the Nikon 14-24/2.8 + Novoflex G EOS Adapter.
On my 5D Mark III, 1 copy of the Canon 16-35/2.8 & 2 copies of the 17-40/4 needed f/11-f/16 to get the edge to edge sharpness that I get on the Nikon 14-24 at f/2.8.
I don't think any of the Canon wide zooms that I've tested have ever, at any aperture, gotten as sharp/even across the field as the Nikon 14-24 by f/4-f/5.6 (b/c diffraction starts limiting resolution at those high f-stops, especially on shorter focal length lenses).
My 17-40/4 was sent to Canon FSC b/c the left side was severely softer than the right. It was softer than the right even at f/18. When I got it back, it appeared to have been fixed (though still required f/11 for the kind of performance I see on the Nikon 14-24 at f/2.8 ). A couple weeks later, the right side became severely soft compared to the left. And it had just been sitting in my bag at home in between (no drop, no abuse, nothing).
So I wonder if it's both a design & quality control issue.
I gave up & went for the Nikon 14-24. With Canon's stellar Live View implementation, it's a cinch to focus. Auto-exposure even works just fine (no Auto ISO though). The Novoflex adapter gives me aperture control.
Here's a comparison (left edge, center) of the Nikon 14-24 at f/2.8 (left) & Canon 16-35 at f/11:
View full-size here: http://cl.ly/G3k9/Nikon14-24_vs_Canon16-35_Left.jpg
And here's the right edge, center:
View full-size here: http://cl.ly/G3bg/Nikon14-24_vs_Canon16-35_Right.jpg
The Canon lens is at f/11 b/c that's the aperture at which at least started to look sharp at those edges. There is more flare in the Canon shot b/c the sun had sunk lower, but I think you can still easily see the sharpness difference.
So, in a nutshell, I'm not at all surprised by your results. Others do make a valid point though that you should check if this happens with all your lenses (better to test similar focal length lenses... especially wide-angles)... if so, then your sensor or lens-attachment ring could be imbalanced.