I'd be more eager to burn a BD25 compared to resorting to AVCHD. How about subtitles? The audio for the main English track for the Blu-ray should be PCM or highest bitrate feasible. Perhaps the commentary as a 448kbps DTS stereo track.
For the BD25 be sure to match it as close to film as possible and use AVC with a good bitrate thereby leaving in the film grain. What about subtitling the commentary and all the video bonus features?