Please see the in-depth Feb 12 discussion Sony’s 11+7-bit Delta Compression, Posterization in Some Situations.
Alex Tutubalin of RawDigger has posted an excellent explanation of how the Sony compression scheme works in RawDigger: detecting posterization in SONY cRAW/ARW2 files.
Version 1.0.5 of RawDigger now includes a special display feature for detecting areas of posterization in Sony ARW raw files. The visual indicators shown by RawDigger can raise concern of image quality, but with nearly all images the eye cannot detect a visual issue. (and it is not easy to find examples in regular shooting).
Aside from a few things like star trails, this particular issue doesn’t bother me so much when considered in practical terms, because it is so unlikely to occur in most images (but if you’re shooting the “wrong” thing, then it occurs in all of 'em). Much more bothersome (unacceptable) is the shutter vibration with the A7R.
The rumor mill suggests a possible “image quality improvement” coming in a firmware release: while it seems unlikely (baked in file format), it’s possible that Sony might do something about this, e.g., allow some other file format. Emphasizing my skepticism for such an option, the ideal scenario would be a 14-bit lossless format, like Nikon. Or at least a 12 + 9 bit format.