As a data point, I have a Caldwell rear bag filled with the beany baby beads and it weighs almost exactly 1.5 pounds. Although I'm very happy with it's performance, I'm always on the lookout for ways to lighten the pack. I've considered filling the rabbit ears with that expanding foam you use around windows etc. , since a little stiffness there wouldn't hurt. It might drop 2-3oz from the weight?
Keep us updated with your findings.
If some is good and more is better, then too much is just right.
My mind is a raging torrent, flooded with rivulets of thought, cascading into a waterfall of creative alternatives