The recent releases of DxO's Photo Lab and Pure RAW now include Prime XD, there new noise reduction with extra detail. Last night I decided to push my limits and find my limit. These two samples tell the story and represent what I found in multiple images.
ISO 10,000 seems to hold up
X-H2S    NIKON AF-S NIKKOR 500mm f/5.6E PF ED VR lens    500mm    f/5.6    1/300s    10000 ISO    +0.7 EV
ISO 12,800 starts to louse detail. Look at the eyes and dark part of face for lost detail
X-H2S    NIKON AF-S NIKKOR 500mm f/5.6E PF ED VR lens    500mm    f/5.6    1/125s    12800 ISO    +1.7 EV
Morris