Thanks for your comments.
I find focusing with this lens (and the 21mm) to be quite easy in all conditions, especially if I can stop it down to 5.6 or more. DOF is quite deep at these focal lengths. With the 55mm and 135mm, I get a high hit rate from f2-2.8. Wider than that I prefer to use another focusing screen (Ec-S on the 1DX and Eg-S on the 5DIII). On this trip I did bring a 5DIII body, which had the Eg-S focusing screen. This screen is made for the 6D and 5DII (I think) and is not a perfect match for the 5DIII. And it may have gotten a bump or something, because it became impossible to use. So these are all shot with a standard focusing screen on another body. I do use the in viewer focus indicators though.
On the shot of Aldeyjarfoss, the colors are very close to how they appear in real life. I use Lightroom 5.5 and have lifted some shadow areas and deepened orange a bit, to improve visibility of the rock structures. Sharpness is set to 50, clarity, vibrance and saturation set to +10.
I did get a comment from a friend that I had overdone the green a bit on the other pictures. He was surprised to see that I had actually lowered both green and yellow luminance, to make it less screaming. The green is extremely green at this time of year on Iceland. Using the polarizing filter helps though (even though it carries a price tag of a decent lens ...)