In the professional tapeless world, the general principle is to make a disk image of your original card, keeping its exact structure. Disk Utility will do this for you. As you say, the AVCHD files take up much less space than anything you can convert them to - but this is largely because of the massive interframe compression being used.
Interestingly, I recently hit one situation where I absolutely needed FCP's import to look at the files as a whole card, and not file-by-file (as is being asked for here): for certain AVC cameras, if a shot exceeds a certain duration, a new file will be created using spanning. That new file may not have a consistent timecode or even proper sync, and must be referred to the previous file for correct continuation. Maybe not a situation that most iMovie users are likely to encounter.
Matt