I have played with a few A900 raw files.
The problem is ACR, and has been for a while now. The level of artefacts produced, is probably down to the base level NR processing going on, I get the same effect with other cameras, though it's more noticeable on some of the sony ones, and some other makers too.
I know some may not like that, and I don't want to spoil the party here, but until somebody takes not at adobe, you are not going to get decent quality higher ISO raw conversions. Broken record or not, it needs to be repeated.