I'm using Subal FE4 with 16-35 without diopter lens, and I'm happy with it.
In 16-19 range the picture is very sharp in the center, but a little bit soft at corners, same at top-side.
In 20-35 range corners are ok.
I'm also very happy with autofocus performance thanks to 2.8. Had a chance to compare it this year at Cocos island: my friend with the same camera 5D2, but 17-40 lens had a lot of trouble focusing at 35m on hammerheads with clouded weather at the top. For me it was very simple, no trouble at all.