The two images below tell the tale - they were not taken together and don't have the same framing, but I think you can see that there is some definite softness to the first image that isn't there with the second.
For the first image I shot at ISO 100, 12 second self timer, live view, 1/25 scond.
For the second shot, I upped the ISO to 800, giving a shutter speed of 1/400, both at f16.
I later did some indoor work and know that a 5 second exposure is absolutely sharp. For now, with the 300 mm. lens, I'll try to avoid anything lower than 1/400 or higher than 5 second but perhaps test two seconds. I'll use the great high ISO capability of the Pentax 645Z to keep away from that area.
This is far from ideal as that's a 10 stop difference or ISO 100 to ISO 100,000 - just not on. Printing the centre section of the 'blurry' first image for a print of 30X40 shows slight but acceptable softness checking from 10 inches. Additional sharpening within Photoshop results in a 30X40 print of superb quality and no signs of sharpening.
This really is the difference with the Pentax, not that it's so many more pixels than the Nikon 810, but that the files are so robust. As far as I'm concerned, I've just eliminated the one nagging doubt about the system - how would 300 mm. perform - very well thank you. The 12 second self timer has fixed 90% of the blur I was seeing in previous shoots outside, and I can work around that other 10% if need be, most of the time, and when I can't, it won't actually matter in a 30X40 print - how cool is that?
I have never felt as good about a system as I do with the 645Z, not even at the time, when I didn't know any better, and now that I do, wow...