At least with the beta firmware, the GH3's ALL-I codec does not seem up to the task, & it's AVCHD looks much better. Some of the more scientifically inclined warned that 72 Mbps wasn't sufficient for ALL-I, & they may be correct.
I don't believe the 50 Mbps IPB has been enabled yet, but it may be the best codec available until the cameras get's hacked.
Even though the 5D3's ALL-I codec runs at fixed 100 Mbps it too has issues, & many users prefer it's variable bit rate IPB codec.