The Sony sensor is a real heavy-hitter that doesn't compromise in resolution, DR nor high-ISO performance, so Leica has some proving to do if they wish to charge $25K for something other than brand name.
I am sure the Sony sensor is very good but for a lot of people other things like the photographic experience are just as important.
I have never used a Pentax 645Z, so I can't comment on that one, but in the 35mm and APS-C world the choice between Leica and Sony is, at least for me, an easy one, despite the sensor edge of Sony.
As always your mileage may vary.
Any sensor without a bayer filter over it is going to perform significantly better than with. The old monochrome punched well above its weight just because of that. I'm sure a monochrome 645Z would have immensely high sensitivity levels.
Again I am sure you are probably right but Leica did have the balls to release such a camera and most others up till now didn't... Credits to Phase One also for their Achromatic.