Hi Peter,
The OP asks for advice about the Leica SL and the Pentax 645Z. It may be interesting to know that Leica uses at least two different sensor designs, Leica are a bit tight lipped about it.The OP may not be interested in DR curves but it is at least exactly the cameras he is asking about.
I also make the small point that most good lenses are big. Interestingly the Leica M lenses are both good and small - but Leica M is not what the OP is considering. He is happy owner of the Pentax 645Z but considered the Leica SL - to save weight.
I do post a lot of images, where I think there is a relevant context. But I don't think web size images give a lot of useful info.
Regarding that anecdotal test, it is not that anecdotal, as it is published by the lady who made it and commented on by the participants. Lensrentals is a very significant operation with over 100 employees renting out a lot of photographic gear so they have some experience with stuff.
Just to say, I have actually bought at least two cameras at above 10k without ever touching them before buy. The Hasselblad 555/ELD and P45+ I bought from Poland, and I have never seen any digital back in real life before that. I have been lucky on that one, I have been told by a former P45+ owner that my sample is better than the P45+s that he used (he had some loaners during repairs). That former P45+ owner now shoots the Phase One 100 MP back and Pentax K1.
The Sony A7rII and a bunch of lenses was also about 10k. I checked out all tests and downloaded the manuals before buying, but I was waiting for that camera for several years. It is more about feature set than en emotional relation.
Whatever tool you get you need to learn it and you don't learn by having a dealer demonstrating it.
In a way, it is interesting. Those guys doing testing have some experience with a lot of gear and they often publish usable raw images and some sites also publish measurement data, but all that information is worthless compared to dabbling with a copy of the gear at a camera shop with a salesman trying to sell the camera.
That said, I guess that Capture One and Digital Transitions actually offer good advice, but they do that in the US and not in Sweden where I happen to live.
Just to say, I have been dabbling with a Leica S in a Leica oriented shop in Luxemburg. I have not been the least impressed by it. Does it make it a bad camera? I don't think so. We also looked at some decent size prints and noted a good one in their exhibition space. The guy who made it said that exactly that shot was with the Canon 5D in some variant. You can make awesome good prints from any modern gear, it is more about subject, composition, illumination than gear. On the other hand, placebo effects are very strong. (*)
As a side note, the new Hasselblad X1D may be the camera he is looking for, same sensor as in the Pentax 645Z but small and with a couple of very good lenses. Priced on level with the Pentax 645Z and Leica SL. Uses the same sensor the Pentax 645Z the OP already owns.
Best regards
Erik
(*) In the "anecdotal tests" at least one of the prints had some colour fringing, quite impossible on the Leica Monochrome that is a B&W device. The Leica users noticed it but choose to ignore it as the "colour fringed" image did have more of the Leica look.
A parallel is that thee were very strong options about CMOS colour when the Leica M (typ 240) arrived. A Leica dealer in Miami, Dave Farkas, posted a three part series of images, having readers tell which was which. Here is the summary:
» The Great Debate: CCD vs. CMOS – Part 3