The “trick” is to import the media from the Media Browser and not import from the file menu, drag and drop, nor short cut CMD-I (ctrl-I) because you want Media Browser to make a new AVCHD content file.
Moreover — and here’s the grading issue — the camera is not only heavily compressed but also aliased, meaning 4:2:0, so any missing information simply does not exist and the computer will have to interpolate that information.
The field recorder is only a benefit if the output signal is better than 4:2:0.
Having said that, another great workflow is to use Prelude to ingest the footage. In there, you can choose from a variety of codecs. For example, Apple Pro Res 422 or Avid DNxHD and so on. The difference here is that these codecs are intraframe, so they will take up more space on your drive, but they are easier to process for the computer in the timeline. If you are moving from your compressed video to APR422, you will not gain color dynamics by using HQ setting, so stay with LT to gain computing efficiency.
For me, I usually transcode to APR422 if I have a lot of footage to wade through, and I know I do not want all of it. There are a several Prelude tutorial videos on adobe tv and you can find a workflow for you.