See a crop of "my" sky in original size here
This seems to look only slightly better than yours.
Only thing that I would control is the jpeg-quality that you use: It should be set to fine and also check with your software, that it does save the jpeg files in best quality. Perhaps we're seeing a bit more compression artifacts in your photo, not noise!?
------ 10 minutes later -----
Ok forget my last remark: jpeg compression artifacts are more blocky not so finely grained.
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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