[johndo]

Plex "works" because the iMac is doing all the real work in this case. Plus, you have to transcode your video which means a decrease in quality. Probably not enough to be really noticeable, but a quality loss none the less. As you already know, the downside to Plex is that it requires a server (and is often not forgiving of older hardware that can still do the same tasks). XBMC is a full blown install, doing everything on the ATV2 itself.

Also, Plex is only sending the ATV2 a 720 video signal (since it's transcoding to the ATV2 client), even though your source file is 1080. That means less data going to the ATV2, less chance for buffering, etc.

Like someone else said, search the forums. There are some good solutions that are working for a lot of people and will probably help you out as well, but in these early days you might have to dig a little bit. Development of XBMC on iOS is still on going, so improved performance is a pretty safe bet.

Good to now! I didn't notice the downgrade.. I already did some search and didn't find something relevant for me.. Now I wonder if it's simply impossible to play some 1080p.. Or Is NAS with UPNP the solution? Or An implementation of an AFP client in XBMC (many people noticed that smb could be the problem)?

I'm surprised you can get 1080p to work at all using wireless. Switched to fully wired and that will help enormously.

I just switch to a wire solution, I connected Time Capsule and ATV2 through a wire and I still get a buffering : slow motion video and always buffering.

I'm lost!