I bought a DA560 new in 2021, probably one of the last ones ever made, and would probably still be using it if it were not for an unfortunate sequence of events.
I now shoot with the DFA150-450 with a TC, and couldn’t be happier. In fact for me, it is a better option.
It is true that the DA560 is a good lens, and it can resolve a lot. I went as far as to post a shot of Jupiter with 3 of its moons, using the DA560 and a K1MKII. I shot almost exclusively hand held, in Tav at between F8 and F11 and 1/1000 shutter.
While it did vignette a little it was usually not an issue because when shooting wildlife (mostly birds) it is rare you are not cropping a little.
Did it purple fringe, a little, but my test was pretty tough, a turkey vulture perched on a steel roof against a bright sky
The absolute biggest drawback against this lens, and it would have been the same for the Sigma 500, is the minimum focus distance, many times I missed shots because I could get too close to focus. That seems like a small thing, but it is not.
On the K1 I noticed very little if no degradation on images between the 560 and the 150-450 plus TC.
The one regret I had was never getting to try the DA 560 with the K3-3 or with a TC, but it just didn’t work that way for me.
If Pentax/Ricoh were ever to do another big fast lens, a 300/2.8 would be a better option, paired with a 2x TC specifically optimized for that lens.
The TC has benefits beyond double the image size, it does not alter the close focus performance of the lens it is attached to , so a 300/2.8 plus TC is more useable than a 600/5.6.
Aside from close focus the other downside of the DA560 was its size. Although people refer to the DA560 as a telephoto lens, it isn’t. By definition a telephoto lens IS NOT a long focal length lens, but a lens whose physical length is shorter than its optical focal length. It was impossible to carry the DA560 in any commercial bag, with or without a camera attached. This made it impossible to travel with except by car. Even my tamron 200-500/5.6 could fit in a lowepro phototrekker 100 bag with a body attached, and I thought it was long and unwieldy
A 300/2.8 plus TC is a nice compact option to 600mm. And I think there is a lot more market for a 300/2.8 than a 600/5.6 or faster.
Until then I will happily shoot my DFA150-450 with and without TC.
The other question of course, is is an ultra fast super tele lens even relevant today? These lenses were born out of a time where ISO400 film was questionable for grain, and AF performance was something that demanded good light. Although people are still in a mindset like this, I shoot an awful lot on dark cloudy rainy miserable days, I let the ISO on my K3-3 go where ever it wants/needs
I recently happened upon an elderly gentleman who used a baby carriage to transport his 800mm. Canon lens, and tripod around while birding. I have seen many like him, maybe I some day will be joining him, who can say. But that is not the interesting point of the meet. He was walking away from where I was headed, having never taken a shot because the “light was not good enough”. Maybe he already had enough shots of what ever he was looking for, I can’t say. But isn’t poor light why fast lenses came about in the first place?