iMovie '11 and earlier will transcode AVCHD into AIC, greatly increasing file size, reducing video quality and wasting a lot of time in the process. Though the new iMovie 13 can handle native AVCHD without transcoding, but the AVCHD clip play back from the timeline though may very choppy. You must have a fast GPU in order to effectively edit AVCHD or it will be annoying. So I think a solution to transcode AVCHD clips to AIC wihtout quality loss and size enlargement at fast conversion speed is a best option for iMovie 11.
http://www.aunsoft.com/mts-converter-mac/
