I was eager to see how my new 16-35MKII compared against the Zuiko 7-14 (14-28mm effective). In terms of vignetting the Zuiko is superb, none at any focal length or aperture. The Canon fell off at 16mm at 2.8, 4.0, and 8.0. Zuiko won hands down.
The perspective of the Zuiko lens is more interesting. At 8mm (16mm effective), it was notably wider than the 16mm Canon...go figure. At 7mm, it was considerably wider, as would be expected. I would be interested to see how it compared with the new 14mm from Canon. So from a creative standpoint, the Zuiko definitely gets the nod. Distortion from the two was both very good.
Sharpness...OMG!! The Canon absolutely massacred the Zuiko. The Canon 100% crop at f2.8 was vastly superior to the Zuiko native at f4.0. It was literally like comparing an L-prime to kit zoom. I was very surprised. I was never floored by the sharpness of the Zuiko, but I am amazed at the sharpness of the Canon.
I was actually considering keeping an Olympus body to use with the 7-14mm because I really like that lens. However, I was not excited at all about having two systems. I used a 16-35MKI some time back and didn't care for it. Outside of the extra width, most of the deficiencies of the Canon can be corrected in Photoshop. However, the detail and sharpness of the Canon lens is vastly superior. Granted, the Zuiko was mounted on a 10MP E-3 and the Canon on a 1DsMKII, but that's the best Olympus has right now. Also, I do get very sharp shots with the E-3 and the 35-135/2 zoom.
I don't have a 17-40/4 to compare with the 16-35/2.8, but the sharpness of the 16-35mm at 16mm and 2.8, 4.0, and 8.0 puts it in the absolute winner category for me.