There is no one simple answer to what is the highest usable ISO for the K-r or the K-5
I have seen shots shot at max iso (51,200 iso) with the K-5 that are incredibly clean after skilled post processing.
A lot depends on the scene being shot - if does not have a lot of shadow areas, and it has an overall low dynamic range, then it is more forgiving.
Next is how the file is processed. If the shot is shot in RAW, and the RAW file is processed very carefully using very good noise reduction software, then you can make very high ISO files look good.
Careful tuning of the color, tone curve, contrast, sharpening, will also greatly enhance the overall look of the final output.
For me, as long as the shot is focussed correctly and exposed correctly, shots at up to 25,600 ISO on my K-5 are salvageable with very careful processing. But of course, the best is to avoid using such high iso in the first place.