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I agree with Grantland's d3s experiences. I've not used a d4, but ISO 1600 on the d3s is not all that different than ISO 200, to my eyes, assuming that the exposure is correct and not needing any push in post.
I'd have given almost anything to have had d3s performance about 7 years ago, when I was trying to shoot high school indoor sports with the d200.
FWIW, I use the low NR setting for high ISO shots, Jorge, on both the d3s and d800, among others. So, I can't answer your question as to how much the image deteriorates at the high NR setting. Sorry.
Hope you get someone to better answer your question.