I've had this lens a short while, and while I can't offer any hints wrt the K-3iii specifically, I can recommend it. The 150-450 is
the multiuse supertele lens for Pentax.
Originally posted by Robert N Is the stiffness of the the zoom ring problematic, either using with a monopod or hand held.
I find the zoom ring isn't really stiff - and it moves by itself if you point the camera up or down. Whether this is an issue or not is up to the individual, personally while it's a tad annoying, I wouldn't really mark it against the lens.
Originally posted by Robert N With the enhanced SR performance of the K-3iii, what is the minimum shutter speed with a monopod to get sharp images of static objects at 450mm focal length.
With my KP (and the K-3iii has supposedly better SR), I can get it pretty low handheld, so I wouldn't be worrying about it too much. It depends on technique really.
For example, this is shot wide open at 450mm, testing how low I can go handheld (without bracing on external objects, like a doorway) at 1/45s - and I'm sure I could go lower still, especially with a monopod to help brace the weight.
And at a 100% crop, I'm not seeing any motion blur:
Originally posted by Robert N Similarly, what is the minimum shutter speed with a tripod to get sharp images of static objects at 450mm focal length.
With a solid enough tripod, there really isn't a minimum shutter speed. If everything is solid without vibrations, you could theoretically take a photo forever. Until an earth quake and then you may get some image shake