In your current workflow, do you use a clap or other sharp noise to give yourself an easy point of synchronization? That's my first recommendation.
There are not very many video cameras with excellent audio circuitry and audio inputs. The Zoom Q3HD and Olympus LS20m are the low end cameras with clean audio and 1080 HD video. The Tascam VH1d shoots 720p HD. The Q3HD is out of production and the Q2HD does not have audio inputs. With one of the first two you could use your Olympus recorder headphone output to feed the audio into the camera so syncing would not be required.
You can try this same trick with your Zi8 although I found the audio captured this way to be pretty marginal.
http://www.homebrewedmusic.com/2010/...the-kodak-zi8/
http://www.homebrewedmusic.com/2010/...the-zoom-q3hd/
High end video cameras, that is, those over $2000 or so, often have XLR mic inputs which would allow you to connect high quality mics directly.
With any of these solutions you could tweak the audio in Audacity or some other program (perhaps even Garageband) and it would line up in iMovie without any effort.
I'm surprised that iMovie doesn't offer any audio treatment features, many video programs do. I use the same VST plugins in REAPER and Edius, for instance. I'm not up on software for the Mac but I would think you could find a program that does audio as well as video. So my workflow with the Q3HD is to connect external mics and a preamp then adjust the audio at the same time that I trim and tweak the video. All the videos in this playlist were handled in this way:
https://www.youtube.com/playlist?lis...PH_Yw_6oVibMA3
Fran