If this is the case, then sadly it’s probably time for a new lens, maybe a Nikon if you can afford it. Lenses should be their sharpest between f8 and f11. I think only being able to go up to f 6.3 is not good enough for many landscape DOF situations. And lens quality issues are only partially helped by post-processing, but not really solved. Spend the money on good glass before bodies or processing software.
I’ve been busy in the evenings getting stuff together and testing a couple lenses. With a lot of reading and great information from http://www.bobatkins.com/photography/ I’ve made some progress. I built the same board below with his updated resolution test charts.
Findings on my Sigma 17-50 f2.8 and my Sigma 70-200 f2.8 are almost identical. They are acceptable between F3.2 and f10, sharpest at f7.1. At a quick glance images at f5.6 appear to be the best but it’s because the image has better contrast not necessarily better focus.
I don’t think at this price point and performance there is anything to complain about. Better focus at a lower apeture can be an advantage for low light situations. DOF may sacrifice some near focal distance but from your subject to infinity I think will be fine. Bottom line is I was trying to use f13 and it wasn’t a good choice. Still lots to learn and more to come.