Hello again! Some feedback from my experience with the lens and the K-5IIs. I'm interested in aviation photography that's why I bought the lens. I had the chance to try it in 2 events in various conditions with and without the HD 1.4X TC.
This lens is surely very sharp and much brighter than the Bigma 50-500 although it underexposes a bit. The contrast and the colors are great also. My pictures of still subjects and slow moving planes and people on the ground were superb and only when I was intentionally shooting with very slow shutter speeds I had problems with motion blurred shots.
In the air though the situation was quite different! I was shooting TAv mode with SR enabled, shutter speeds 1/1600 to 1/2000 (fast moving jets performing aerobatics) and aperture f/8 or f/9. The AF set was continuous and auto (11 AF points). From what I've seen so far the pictures are not tack sharp, it seems that something is missing or something interferes with the lens capabilities and degrades the pictures!
I don't know what could be wrong, but since with the Bigma gives great sharpness (although the lens isn't as sharp as the DFA and the ISO is much higher) I presume that the problem should relate to the camera SR which is the difference between the two systems. At the end of the day I had less keepers with the DFA than the Bigma and only a few that were top class pictures. I was negatively surprised as I told you I have no doubt that the Pentax lens is BETTER than the Bigma and it has no front or back focus issues. I will make further tests with various settings in order to see what works best. I am thinking of trying:
1. Slower shutter speeds. Maybe the SR will be more effective if it has more time for "calculations"...

2. Even faster shutter speeds. That means that I will go over the 1/2000 that I had rarely reach in the past (as it was not necessary).
3. Deactivate the SR. I'm not sure if I could get sharp images of fast moving jets without SR @450mm, but I will give it a try and find out.
4. Change the AF to single central point or to just 5 AF points...
A friend of mine has a K-5 so I will try to find an opportunity to shoot the Bigma and the DFA side by side at the same conditions and see the results (although the cameras have some differences, it is the best comparison I can organize).
When I find some time I will post some of the good pictures for you to see. Meanwhile if you have any ideas or comments on the shooting settings or the tests I will be glad to know!