OSXBMC rarely drops frames in 1080p my my mini HTPC. Its ffmpeg libs underneath, however, so the results will probably be no diff from mplayer.
that's very strange ... i wonder what professional editors play their 1080p videos on using MAC i know a lot of HD editing is done on MACS so what r people using to play these videos?
try Mplayer from the command line. Your computer will rock.