I received my HD TC yesterday, in time for my trip to Puerto Vallarta. I am very impressed with the K-3/TC/DA*300 focus performance. I won't hesitate to use this combo for BIF.
My next test was to see what would happen if I mounted the Pentax TC with my Kenko 1.5x PZ and DA*300. I am pleased to report that, as long as you mount the Kenko first, the combo autofocusses pretty well (I only tried in good light). Much better than the Kenko and DA*300 alone.
The K-3 does not see the Kenko, just the HD TC and lens, therefore aperture and focal length are underreported, but the IQ was pretty impressive. Certainly a lot better than my Pentax old A 2X-S. (Sorry, no samples, I don't have time before the trip).
EDIT: See disclaimer here
https://www.pentaxforums.com/forums/16-pentax-news-rumors/250767-hd-teleconve...ml#post2751300