Not completely relevant to the thread, but I said I'd follow up. Like the 17-40mm f/4L, the 16-35mm f/4L IS has no vignetting at 20mm and above with an B+W XS-Pro clear filter, wide angle Lee adaptor, Lee filter holder with two slots and a 105mm adaptor, and a B+W 105mm extra wide circular polarizer. My 24-70mm f/4L IS is good at 24mm.
I'm curious whether you can go below 20 mm with either the 17-40 or 16-35 f/4 if you take the clear filter off before attaching Lee wide angle adaptor/holder/105mm adaptor and B+W 105mm extra wide CPL (I take it this is the one that is 105mm on the attaching side, and larger on the side towards the subject)?
It would be useful to know for those who do go bare when using the Lee system (and I have been one of them - although I confess I have been using the CPL between lens and Lee and have gotten it to work sortof - not well).
I did a quick test on my 17-40 with Lee holder, 2 slots, with 105mm ring attached. Oh and to answer the question what 105mm cirpol I'm using look at the picture
All images shot at F8 against a whiteish wall, I've overexposed them somewhat to get rid of the shadows on the wall. The ones taken with the cirpol has a four time longer shutter speed to account for loosing about 2-stops with the cirpol.
First image is taken at 17mm with just the filter holder attached to the lens, so there is no vignetting from the 105mm filter ring attachment.
Image number two is at 17mm with the cirpol on, obvious vignetting.
Number 3 at 20mm with cirpol, slight vignetting in the corner.
Number 4 at 21mm no vignetting visible.
Edit: Oh, the order got messed up, but I guess you can read the filename for each image. Sorry about that.