>>3800727 (desaturate greens quite a bit)>>3800729 (v. nice environmental portrait)Looks pretty good to me. Sun isn't necessary an advantage. What you don't have is the best snow. When snow is still clinging to the branches, when everything is still fresh. In that situation, every snap usually is golden, sun or no sun, all day long. Early morning, after humid night can give you some nice frosted looks also.
As for 16-80 f4, that's a pretty standard range for landscape. I don't think prime would allow you to take better shot, but it would possibly make you reconsider your shots more. Usually the next landscape lens to buy is a nice zoom lens in 50-200mm range that allows you to isolate subjects.
