OK, here's another quick and dirty test. Taken in the back yard. A cold sunny morning, in which the thermal effects were enough to make the image a little woozy, so though not ideal for sharpness, it's a real world sort of day. D7100, on a tripod, ISO200, 5.6 aperture on lens, Aperture priority, shot with self timer. The converter is a Nikon TC-14. The crops are done and converted to JPG in View NX-2, conversion and downsizing of uncropped original, and juxtaposition of crops, in Irfanview. No post processing of any kind.
In the comparison, left is without TC, right is with.
As you can see, the image quality is not much different, but interestingly, along with getting a less pixellated image from the TC, it looks as if the chromatic aberration is less with the TC.