I posted graphs for PS3 and BDT110 in stereo back in this post; in this post, I estimate the difference between stereo and DD+ 5.1 to be about 75 Kbps.
For some reason Ong Bak 2 doesn't show up as having 5.1 audio in the Roku's GUI; no audio option is shown at all, just a subtitle one, so I think that's a UI bug. I could hear the soundtrack so it was giving me the stereo one; curious that it used an almost identical amount of bandwidth as the PS3. Whatever.
(BTW, you just vindicated my decision to wrap all of my inline graphics and long text charts in [spoiler] tags. People who quote four pages worth of photos and make me scroll through them again just to read their two line response drive me nuts. People can quote my media-ladened posts as much as they want as long as they leave my tags in ).