Did you conduct this test because you have taken a lot of photos with this lens that look like they have soft corners, or just to satisfy your curiosity?
The reason I ask is because I'm curious if this lens is showing obvious image quality problems in regular photos. Sure, it might not be the best lens for photocopying pages of text, but that's what a short macro prime with a stable tripod or copy stand is for.
For example, one of my favorite lenses is the Sigma 24-70mm f/2.8 DG ... which I have purchased for THREE lens mounts over the last 4-5 years (Nikon, Canon, and Pentax) and is my "go to" lens for most of my work. The Sigma was also the only lens that I chose to bring with me to the hospital when my daughter was born.
My point is, as much as I love the Sigma I doubt it would perform any better (and probably WORSE) in this type of test than your DA* 16-50mm.
We tend to pay a lot of attention to "test photos" when a new lens comes out, but in my mind a lens only has serious problems if things like lack of edge sharpness, poor contrast, inaccurate color tone, and harsh bokeh are obvious in typical/real-world photos.