AMA continues to be a hit & miss proposition with many factors involved form drive speed, connection speed, length of source clips, and codec. Combined with editor expectations of number of layers (streams), duration of program for VFX,multicam etc., many will find that a transcode to DNxHD still may be the best solution. Some can edit AMA all the time and only transcode finished sequence, many cannot.
When you say "import" do you mean File->Import or an AMA link and transcode?The latter is much faster as it multithreaded and uses more cores.
As far as FrameFlex and re-frame extraction from the 4K image, you can try to stay AMA the whole time to take advantage of that, or if you transcode, do not check "legacy" when transcoding. This keeps the "metadata" active so that even if using FrameFlex to resize the event so when you relink/conform back to the camera originals, you can take advantage of the extra resolution. Just be aware of conform/transcode as it seems to be a logical method, you will want to render the timeline once conformed in order to maintain the expected quality. Something I blog about here: http://24p.com/wordpress/?p=160