I use 5DtoRGB which transcodes AVCHD files from Canon and Panasonic HDSLRs (most I believe). It's free to download, was originally written for mac, and transcodes to prores at a high quality (I find it to be slightly better at replicating small details then when footage is transcoded using Final Cut itself). The file size of ProRes is quite large but is "visually lossless", if you try to convert the footage to a more lossy codec you're going to start losing a lot of information, hence the name.
With Final Cut 6, 7 and 10 you can also transcode GH2 footage straight into Final Cut with log and transfer. I don't know if FCP 5 is any different.
I modified the batch script to open .MOV, .mov or .MTS. was not too hard. Just change the line if fname[-4:] == '.MOV': FILES_LIST.append('%s%s' % (input_path, fname)) to if fname[-4:] == '.mov' or fname[-4:] == '.MOV' or fname[-4:] == '.MTS': FILES_LIST.append('%s%s' % (input_path, fname))
Have you tried re-wrap avchd?...Copy your files before you use it to be safe...I had little luck with it myself but maybe in your case it will work. FCP should let you import the re-wrapped files . There should be no major quality loss. Just get the Perian QT plug-in its free too. And its great to have on its own right.