If you want to stay at 1920x1080 its a bit of a hassle, and it requires transcoding everything. Thats the nature of AVCHD tho and not really iMovie's fault...
I do all my iMovie at their 'large; setting, but I keep a copy of my native 1920x1080 files for future editing by a drag and drop process, it treats the cam just like a hard drive...