Lenstip has published some comparison shots between Panasonic GF5 and Panasonicat various ISO levels. The performances appear to be similar up to ISO1600 but starting at ISO3200, you can clearly see the differences. ISO3200 looks very usable. Even ISO6400 might be ok if you are desperate.
Nice performance out of the new little 12MP sensor.
Source: http://www.lenstip.com/1984-news-Panasonic_Lumix_GF5_vs_Panasonic_Lumix_GF3.html












The GF3 has a very similar noise pattern to my GF1 – noisy but honest. The GF5′s more aggressive noise reduction is a little too smudgy.
GF3 seems better in detail