I think I might get this lens as a take along/do it all canoe tripping and hiking lens on the 6d. Have you tried it on a crop camera? I wonder how it compares IQ wise compared to the 16-300? I know it has less range on the wide side but I would prefer it if the IQ is better on a crop body as well.
I haven't posted my review of the 16-300mm yet, but I actually make that case. There are more optical compromises with the 16-300 and I still like the 28-300 better on a crop. Less loss of resolution and contrast on the long end. The biggest downside, of course, is that 28mm isn't particularly wide on a crop.
Here's a sample taken with the lens on a crop camera (300mm wide open). Crop body is an EOS M in this case, but I did the review primarily on a 60D.