Just did some lens tests with the GH3 vs GH2 w/ Flow Motion v2.02 hack. Information below:
GH2 vs GH3 Night Lens Test
Panasonic GH2 w/ Flow Motion v2.02 Hack, Vivid -2,-2,-2,-2
Panasonic GH3, Vivid -3,-5,-4,-5
Panasonic Leica DG Summilux 25mm F1.4
Canon FD 50mm F1.2L
Canon FD 85mm F1.2L
Canon EF 100mm Macro F2.8
Perspective is different even though I used them on the same tripod. GH2 is 1.86x sensor crop, GH3 is 2x sensor crop.
In very dark situations the GH3 is noticeably brighter, about 2/3 of a stop. I tried to compensate by making the GH2 1 stop ISO brighter. But by doing so makes the GH2 have more noise and have "milky" blacks. Color correction is very difficult with the GH2 even with the hack. The GH3 does much better in this regard, although detail might be a little lower.