As a follow up I did some more testing in combination with Jim over on Adobe Forums. Using the same firmware version that he does “Driftwood Cluster v7 ‘Apocalypse Now’ – 6 GOP Nebula ‘444 seti” my GH2 MTS files import and export form PP CS6 without fault so the camera is not at fault as such.
At least we know it is a combination of GH2 stock compression settings and the Main Concept filters used in PP CS6. Since both sides will point the finger at the other the solution seems to be using Pro-Res transcoding or custom firmware.
So no definitive reason for the problem but a better understanding I guess.
Editing is why people like movies. Because in the end, wouldn’t we all want to edit our own lives?