For FCP user having to encode seems the normal way of doing things, but if your computer can handle the original AVCHD then go for it.
Something that I have done in big projects where running multi cam and more than 5 video feeds at the same time is down converting all my footage to mpg1 just to edit, and when Im ready to color correct and export, I just have to relink to the original AVCHD and everything will be just fine.
About the 30fps, my advice is to get some 60p footage inside your sequence and export it on 30p to see if there is a big noticeable difference. If you have a lot of footage, having to transcode all before editing can take a long time.
Maybe a good solution will be edit in a 60p sequence with the original files and after you are done, export the final edit on 30p