Get Nikon D5 at B&H Photo.
When shooting in lossless-compressed raw (NEF), the file size is a directly-correlated metric of image detail, which includes noise (randomness). That is, information theory of entropy says that more randomness (more noise and/or more detail) is less compressible.
Thus, when taking dark frames, compressed file size is a direct metric for the ISO range at which noise increases to more problematic levels. One needs no fancy analysis: just look at file sizes for an accurate study of the ISO values which are likely to be troublesome relative to lower ISO values. This of course does not address pattern noise or other problematic behaviors, just noise overall.
This study is an interesting confirmation of the conclusions in the Nikon D5 ISO series, based on NEF file size alone!
Dark frames shot at 1/8000 @ f/16 with the rear viewfinder blocked, lens cap on and camera covered—no light whatsoever. All frames shot in lossless-compressed 14-bit NEF.