E-M5 ISO Extension
Using the Super Control Panel, when I set ISO to 6400 or higher the word "Extension" appears on screen. This designation does not appear when using the menu to set ISO. (I'm running firmware version 1.2)
I take this to mean that the sensor is set electronically to different sensitivities, amplified in hardware up to ISO 5000. At ISO 6400 and higher, ISO is 'extended' to as high as 25600 by software manipulation to make an ISO 5000 base image appear brighter, even though it is actually underexposed.
Without doing any testing, I've set Auto-ISO to an upper limit of 5000. I'm very happy with IQ of my dimly-lit concert photos at ISO 5000, using the ZD 35-100 at f/2. I'll go higher if forced by dim stage lighting and still get better stage photos than from my E-30 at ISO 1600.