I use my 150-450 sometimes with the Pentax HD 1.4x tele. This combo vignettes heavily on FF but the centre sharpness and AF performance is pretty good, so I do use it sometimes for (casual) shooting of more distant birds (where it's impossible to fill the frame) under good light.
I do wish Pentax would develop a modern FF HD TC. I used to have the 1.7x AF one which was ok, but showing it's age with more modern lenses and not as sharp in the centre as the modern APSC 1.4x.
Anyway, here's a couple of 150-450 shots from me - as usual the birds saw me coming on this walk, and made themselves scarce!
John