I'm pretty sure the Plural Eyes plugins already do that. Not free, sure, but neither is FCP.
Can FCP X import AVCHD natively? I'm pretty sure past versions can't, right? I don't own a Mac, but if I did, being forced to transcode everything to ProRes would be an absolute dealbreaker for me. If I transcode before editing, I do it because I want to do some Avisynth filtering on the footage. In those cases, I edit the AVCHD natively (in Premiere), then, just before export, encode the Avisynth scripts to uncompressed video and do a Source Replace. Having to make ProRes intermediates for everything? No thanks.
Just IMO, of course.