Would be nice to see comparisons with 645z against A7rII. Like I mentioned I talked with a co-worker who had tried 645z and his opinion was pentax glass on that body is not great. This is from some one who has used top of the class stuff. On canon he shoots 1dx and let me try his Sony A7rII which was awes some BTW. Other concern I see is the need to use tripod for lot of situations where I would be shooting HH, even portraits.
The way lenses render is fairly subjective... many of the older Pentax lenses hold their own on the Z, especially when stopped down, although you'll want the new glass to let it shine, one doesn't buy an $8000 camera just to mount $700 lenses on it after all. The updated FA 35mm 3.5, 28-45mm 4.5 and 90mm 2.8 are all top-class lenses that come within 95% the performance of camera systems 2-3x the price, and will surely outclass anything on smaller formats.