Because I was curious, I quickly set up the K-x for a still life shot at ISO 12,800 using the FA43. I set it to f/4 and the camera metered 1/60s. The K-x was used on a tripod with mirror lock up.
Here are two 100% crops, the first untouched, as converted from RAW by ACR with everything on neutral (NR and sharpening off). Since I would never care to look at an image unprocessed, the second crop shows the results after my "usual" PP, though with no special time taken over it. (I might have chosen to back off on the sharpening a tad.) Noise reduction has here been applied using a minimal setting of Noise Ninja. I note that all the in-camera NR is turned off.
100% crop untouched 100% crop processed complete image processed
The final picture is the processed image in full, though obviously resized for the screen. Any artefacts visible in the crop are here rendered palatable. I have no problem with this image, though would never "in real life" resort to such a high ISO.
Also the subject needs some work! :-)