The 16-35 II is compared to 17-40 a big step forward. Particularly as regards the field at f 4.0, there lies the 16-35 II near the front. At maximum aperture, the two lenses do not give much, the 16-35 II is marginally better, but both are very soft! Even the 16-35 II has the usual strong focus on waste from the image center and a Distortion with smaller focal length. However, I still not allowed to own UWW what does not have this weakness, these are the 16-35 II less pronounced than in 17-40.
Overall, the 16-35 II is a very good UWW optics, but can not completely hide the usual problems of UWW lenses. Whether the enormous extra cost is worth to 17-40, depending on the personal budget - for me it has definitely paid off!