It took me a few minutes to do the raw conversion, and I had problems with a magenta cast to images, so hopefully I can save the rest of you a bit of time.I got a similar "view" of the files using IrfanView. I am lead to believe that it is a faulty RAW conversion and that useful claims about noise cannot be made.
Look at a 600% enlargement of the left top corner of the ISO 6400 image to see what I mean. 8x8 pixel "blocks" with articulated edges are visible that introduce incorrect "detail" that will mess with any kind of noise-reduction one might want to perform on it. I can only assume that that is a RAW conversion error.
Are you sure irfanview is not just showing the highly compressed embedded jpg within the RAW file? JPG also compresses in 8x8 blocks I believe (although it shouldn't show itself as what you see so something is probably going wrong).