Hi,
Posterisation, I would say not.
Now, Sony employs two kinds of compression. The first is in essence similar to a gamma curve. At high data numbers they have larger steps. That is basically sound, but it may be that they would have overdone it, I don't think so, and I have not ever seen posterisation in my images.
In addition they have a kind of "delta coding", that can induce artefacts. I don't say I have observed it in my images clearly. I have seen some artefacts, in the very same image posted on this thread, but it is in the wrong direction, I think.
This image digged up by Diglloyd is the best illustration of the artefacts I have seen:
And here is a long article from the "rawdigger site":
http://www.rawdigger.com/howtouse/sony-craw-arw2-posterization-detectionMy take on the issue is that they apply the tone curve so they can put more than 12 bit of data trough the "Bionz" processor which may be just 12-bit wide. I would thing it is absolutely OK.
The delta compression can clearly yield artefacts. On the Sony Alpha 99 I am using mostly there is RAW and short RAW. I always use the larger file format. It may be I don't have the "Delta" compression. I don't know.
Jim Kasson, a real scientist, has done a lot of research on this issue, perhaps a year ago. It was presented on his blog:
http://blog.kasson.com/?p=4838Best regards
Erik
Ps. I have seen that you googled on the issue while I was posting my response... :-)
Do you ever get posterization with the Sony lossless compression data?