ephotozine done a High ISO comparison test between Olympus E-M5 and Panasonic GH3, Both camera was using Panasonic Leica DG Macro-Elmarit 45mm f/2.8 ASPH OIS Lens and ISO at 1600 and other settings set to Auto (AE & AWB), The Olympus E-M5 JPEG output streight from camera is more clear and sharper compared to Panasonic GH3.
source / Image credit: ephotozine