Vimeo user emeric posted this Panasonic Lumix DMC-GH2 vs. Panasonic Lumix DMC-GH3 comparison of ISO results for each camera in the same low light situation. You can see the side-by-side comparisonm in the second half of the video. Check out the full review on www.hybridcams.fr. The post in in French- Google can translate.
GH3 : Panasonic 20mm at f/2.0, ALL-I 25p mode, 1/50, auto WB, Standard picture profile (all at 0), firmware v0.5
GH2 : Panasonic 20mm at f/2.0, AVCHD 1080p24 mode, hacked (Sanity V5 patch), 1/50, auto WB, Standard picture profile (all at 0)