Agree 100% about the 16-35
I like it on my 7D, but love it on my 5DII.
In fact, I sold it, missed it and purchased it again.
+1....7D it was ok, on the 5DIII its a different story. I would keep it until you are able to check it out on a FF first. I'm happy I didn't sell mine after the disappointment I had using it on the 7D.