Recently I picked up a DA 1.4x teleconverter, new on sale. I got it primarily to extend the reach of my DA 300/4, but the paperwork also said it's compatible with the 31 and 77 FA Limiteds. I tried that out for the first time today and got awful results. The edge detail on the 31 was so poor that the blur stood out in the viewfinder. The 77 was considerably better, but still lousy. Only at f16-22 could I get decent edge sharpness. The two lenses perform beautifully by themselves on my K-1. Could I be making some kind of mistake here? This is an unpleasant surprise. Even the staff review here mentions using the 1.4x with the FA 77.
Here's a corner crop of a flat rock face, with the center in good focus.