You can stream 720p with a N router but not a 1080p.
The atv2 WIFI it's limited to 65mbits :/
It's documented all over the internet, so the problem with 1080p and buffering issues it isn't related with XBMC.
With Airport Extreme in the Statistics window you can see other devices connected ate 250-300 mbits and atv2 always at 65mbits
https://discussions.apple.com/thread/273...0&tstart=0
https://discussions.apple.com/thread/280...&tstart=-1