The high-ISO looks good, what I cannot understand: if the High-ISO looks good, and the sensor has a high dynamic range- why not use 14-bit values for base/low ISO and truncate the low-order bits as sensor artifacts creep in at higher ISO. I'm going to guess that the camera is applying a lot of processing to the internal 14-bit image and storing 12-bits of it. So, "out-of-camera" images look better than what you would get from the CCD which stores very lightly processed data. Pass an ISO 10,000 image through LR with some noise reduction to take out the last 2 bits, you get a clean image. The M Monochrom images compare with my Nikon Df at ISO5000 and 10000.
So far, only Jpegs are available and these are 8-bit images. The test comes when you have the raw image on the computer and start applying "curves" or anything else to it.
Okay, I'm mad about the 12-bit pixels. I'm betting Leica could revise the firmware and move to 14-bits for low-ISO, move to 12-bits as processing is applied at High ISO. That's how I would have done it. Of course, hard to know what is locked into hardware and how much flexibility is left in firmware. The difference in the M8 14-bit Raw files compared to DNG-8 is incredible, and the camera went 8 years before someone corrected Leica's mistake.
"Usual Disclaimer", I never have any strong opinions, and even if I did- would never think of expressing them. And this is why I never wait for someone to ask "But Brian, tell us how you really feel about this"...