As for experimentation - have been playing around with high isos recently. Today for the first time tried 12800 (yes, it is 6400 electronically thrown to 12800) but results were not all that bad. Chroma noise lots of it but easily managed. Luminance noise not as easy but still manageable.
Will post results later.
I notice grain from high ISO starting at 800 do you have the compensation on or off