We recently received our AG-AC160P and have been using FCP7 and testing FCPX and Premiere Pro CS5.5. I have done a few test edits with AVCHD only in FCPX and Premiere. I use J,K,L a lot to shuttle around and found the native AVCHD in FCPX to be very challenging, jerky…finally had it convert to prores proxy. That was pretty fast and worked generally well. Then I did the same in Premiere (native AVCHD) and found it much smoother except when trying to shuttle backwards quickly.
This was on a MacBook Pro, i7 with 4 gigs of memory and an external firewire 800 drive (7200 rpm, WD Caviar Black)