>AVCHD files will never be supported by Adobe
I use PPro 5.0.3 to edit AVCHD from a Canon Vixia HFS100 with zero problems... of course, I am a home hobbyist shooting short family clips that I combine on the timeline, so I currently have no experience with >4Gig "spanned" clips
A program bug is not the same thing as "will never be supported"
I do not work for Adobe so I have NO idea what source code changes were made between CS5-CS5.5-CS6 but my understanding is that CS6 added new features, and new codec support, so finding a bug is very likely going to take time