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Your first shot at ISO 6400 is about 1 stop underexposed which is a recipe for noise on an APS-C body. When I shoot at ISO 6400 or 12800, I will always shoot at least +1/3, and alot of times +2/3 to the right. Also, what is your high ISO NR set to? It appears to be either Low or Disabled, since the NR stored in the raw are lower values.
I have processed your image using DPP and then CS3, would you mind if I post them?