Paul,
I believe it's been fairly well demonstrated that the reduction of high ISO noise in D800 files when they are downsized (ie: in photoshop) to approx 12-16MP, does not occur when you simply set the D800 to shoot in DX mode. In camera downsizing of files does not significantly (or possibly not at all) reduce noise levels as seen in high ISO images.
Dave (D&A)
I believe it's been fairly well demonstrated that the reduction of high ISO noise in D800 files when they are downsized (ie: in photoshop) to approx 12-16MP, does not occur when you simply set the D800 to shoot in DX mode. In camera downsizing of files does not significantly (or possibly not at all) reduce noise levels as seen in high ISO images.
Dave (D&A)