It definitely looks noisier than it should. What WB settings did you use? I normally set it to 3700K.
Here is one with the same lens and Nikon D700.
http://www.terrastro.com/wp-content/...the-clouds.jpg
I was checking the composition at ISO 25600 with short 5 seconds exposures and forgot to switch it back to normal ISO 3200 before taking the shot at 15 seconds, f/2.8. It turned out surprisingly good. In PP the only thing I adjusted was turning the exposure down half a stop.