You need to develop a work flow that you like. My canon HD camcorder has the option to record mp4 format video that quicktime can directly work with. But when I do record in AVCHD I use some software called "clicpwrap" that can rewrap the video without transcoding. there is no quality loss. http://www.divergentmedia.com/clipwrap
You can also use "handbrake" but I'm pretty sue handbook will transcode the video. Clipwrape is as fast as a "copy" and I use it rathe than drag an drop to get video off the SD card.
You do have to decide at which point you do the archive. Some people do a rough edit just to cull out the total junk that is out of focus and whatever. I like to archive the video at this point, avetr it is converted to something quicktime can use.
Next I import it to FCP X. This can have the 5x effect you describe but those prores files are the best format for editing. the import/transcode process goes very slow. In FCP X I can turn that off and edit with the original .MOV files if I like. I FCP X I can at any time trash the large prores files. Does iMovie allow this?
OK so then I have an edited vedio based on the ProRes files. The next step is to export it to some usable format for my Apple TV or iPod.
After the video is exported there is no need to keep the large ProRes files around unless I think I might re-edit. Periodically I clear out those large files.
Backup is an issue. Time Machine picks up the .mov files from the camera but I also use other redundant backups and always have at leas one off site backup.