My experience so far airplay mirroring in mountain lion with MBP retina + atv 1080p connected via ethernet to my verizon fios router:
- XBMC: non-HD content plays without stuttering or audio hiccups | HD content (regardless of container: mkv, iso, etc.) plays well for a few minutes but eventually stutters
- VLC: same as xbmc, though HD content sustains smooth playing for a bit longer
- safari: have only tried amazon prime HD. content stutters pretty quickly.
Because of the HDMI port in the MBP, I don't need to muck around with airplay mirroring. Even prior to the HDMI port because the thunderbolt<>hdmi adapter worked exceedingly well. Still, I'll continue to tinker periodically with airplay mirroring.