Received my 1DS Mrk III last weekend down here in New Zealand and have been testing with my 16-35mm MrkII. Certainly I can see a difference in quality around the edges compared to my ID Mark II as one would expect comparing with a cropped sensor.
But in the end it is only in the corners were the softness really shows and at 24mm I was rather impressed after sharpening in Lightroom and adding some "Clarity".
The lens is slightly sharper than my 17-40 and has much less CA. So my advice is if you wish to say with a Canon wide angle zoom the 16-35mm MrkII is currently your best choice. Yes ... and looking through the new super bright viewfinder with an f2.8 wide angle lens is amazing!!!
So my choice is not so clear after all... But if I understand all of the comments it seems that the 16-35mm II should be ok for landscape work with a full frame camera so long as I'm willing to use f8 or smaller.