Going from version I to version II, Canon did make some progress...the fact it is not that stricking a progress is more telling of how hard it is to design UWA zooms than it is of Canon's unwillingness to make it better.
For the same reason, I feel Canon will only get even fewer paybacks with a version III should they ever attempt it. Where they did make progress is with corner and border sharpness and flare control...the original version I was worse. But these are only minimally noticable.
As someone said earlier, the 16-35 II is not super sharp or super perfect...but it is really a well balanced performer for its range for real world uses.
Here is a shot at 16mm straight into the setting sun at f/11 when I was looking for flare. No filters or anything. I am including the top border crop as well...not sure what CR uploading does but it is sharp in the originals.