Nikon D300S High ISO RAW Image Quality
We've recently started looking at RAW files converted with dcraw, an excellent freeware raw converter. dcraw usually offers timely support for the latest cameras, but more importantly, it does not apply any noise-reduction, sharpening or other corrections such as geometric distortion correction to the output files. (We found that Adobe Camera Raw still applies some limited noise-reduction when its NR settings are set to zero, and it also applies other corrections depending on the make and model of the camera). There will always be differences between RAW converters, in terms of the sort of demosaicing algorithms they use (the processes by which they convert the separate Red, Green, and Blue data sets to an array of full-color RGB pixels), but dcraw seems to use a fairly generic algorithm that delivers good sharpness with relatively few artifacts, and can be counted on to not apply any noise reduction if you don't want it to.
Below are crops from Nikon D300S high ISO RAW files compared to RAW files from the Nikon D300, Canon 50D, Canon 7D, and Pentax K-7, converted with dcraw.
It's actually uprising to see how similar these cameras perform in terms of high ISO RAW file noise. The D300S compares favorably to the others in this group, though at 12.3 megapixels, it has the lowest megapixel count. (The Canon 50D has 15.1 megapixels, the 7D has 18 megapixels and the Pentax K-7 has 14.6 megapixels.) That should give the D300S an edge in terms of noise, but that isn't the case. Competing models are achieving similar levels of noise, despite having smaller photosites.
For Nikon shooters thinking of upgrading from the D300, or stepping up from the D90 or D5000, we've included crops comparing those cameras below.
Very similar results here, though we'd say the D90 and D5000 have a slight edge in high ISO noise performance over the D300S.