Louis... I haven't shot the E-M5 at high ISO much, as yet, but this grab shot in my motorhome was at ISO 3200 with the Zuiko 9-18 lens - no noise reduction of any kind has been applied.
I'm not sure if the E-M5 yields as clean high ISO images as the G3 does, but it's close if it isn't quite there.
For me, Olympus has finally put together what has LONG been needed from their brand - a body with built-in EVF, tilting LCD screen, good controls, superb IBIS, and reasonably good high-IOS performance - all in one body. I must commend Oly on the controls on this camera which while not perfect, are very good. This body is a noteworthy high point for the m4/3 format and brings me to a point of transferring over from preferred Panasonic bodies over to Olympus. I will be quite content shooting with two E-M5 bodies and a nice assortment of Pany, Oly, and legacy lenses.
The E-M5 , by the way, by virtue of the superb 5-axis IBIS and the controls, makes using legacy glass really great to work with on this camera!