Just turned up this thread while looking for something else ...
I find that the mFTs sensors, even the 'noisy' 16 MPx Panasonic one in my E-M1 MkI, are very good to ISO 6400, and even quite usable at ISO 12,800 in appalling light. Not as clean as much larger sensors of course (mostly ... ), and I have found that using the RAW file helps a lot, as JPEGs at ISO 12,800 are largely awful.
However, in the RAW files the noise tends to be very fine grained, without blotchiness or sensor blooming that I have seen, even with a FF Sony camera at lower ISOs than 12,800.
Knowing your camera and light are both crucial, of course. Even more so than in 'normal' lighting conditions.
Some examples.
The Eureka Tower, taken from a semi-pontoon bridge with about 100+ people walking on it. Late at night, cold, after a Christmas party (I don't drink alcohol).
Note several things:
1) shutter speed - excellent IBIS helped;
2) fine grain noise in lit windows and on the facade, without too much loss of detail;
3) two stars visible between the crane boom and the building, and one in the upper r/h quadrant in line with the end of the crane boom;
4) this image will print acceptably at A3 size (297x420mm).
My web site image is here:
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Secondly, a few of the cats. These are effectively OoC JPEGs with minimum or no noise reduction (depends on body). All at ISO 6,400.
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Note that two of these were taken with kit lenses.
[Edit] Some more at ISO 6400 here:
[End edit]