So my main interest in the new high isos was to see if any of the standard range isos had improved at all.
Seems there is no discernible difference with the normal range. As for the new isos, you need to enable iso expansion in the menu to get to them.
iso 25,600: not much worse than 20,000
iso 32,000: a notch noiser, but still holds together and could be usable
iso 40,000: significant dip in IQ the lower third of the frame starts to develop an odd greenish band
iso 64,000: green band more prominent, and IQ drop
iso 80,000: Well, it's impressive that the sharpness/detail holds up as well as it does, but the grain is no longer uniform and the green band is very prominent.
i'd draw the line at 32,000, and that's only if you are truly desperate for light, which should be a rare situation seeing as 12,800 and a fast lens can expose even a dark bar just fine. I'm sure some will find use with 80,000... certainly better to have it and not need it than to need it and not have it.

Testing C100 80,000 ISO firmware against old firmware for improvements + high ISO usability on Vimeo

Holy cow, 80000 iso...and I thought 6400 iso was allready an achievement on my gh3. :)