kk650 said:

Do you think that the downmixed BD audio would be noticably better than the downmixed dolby digital 5.1 audio though or is the difference likely to be very subtle?

If I might pipe in, here:

One man's "very subtle" is another's "Night and Day."

For this man, the DVD sound is so irritating that it causes listener fatigue and makes me want to turn it off. The BD Master audio is way, way better and easier on the ears. If you must have stereo (why stereo only?) I'd request a full-res 96k/24bit stereo downmix.