Hi Edward,
A friend had concerns like your partner’s, and needed a very light sleeping bag for a long hike in Europe. We found that some of the bag makers use “no kill” down; that is, the geese are plucked but not harmed, as I understand it. She settled on a Western Mountaineering bag that filled the bill.
Before “featherless” down, 3M came out with a product called Thinsulate Lite Loft that had more loft for weight than Primaloft or Climashield products, although it was a bat type insulation. Was going to make it into a bag with very light .67 oz fabric and stretch mesh from lingerie for the baffles. But the consensus on BPL was that although it might be as good as down when new, repeated compression in stuff sacks would reduce the loft, a characteristic of synthetic fills. So put the project on hold.
Watched a video to learn more about the featherless 3M insulation: https://vimeo.com/249408987
Saw that it is a loose material, like down, and goes into jackets with compartments around 3″ in width. It also does not absorb water. But no word on the effects of compression, although you seem have addressed this issue by your own observation.
Also looked for a bag with the featherless 3M, but no better luck. Just jackets. Since the product has been out since 2014, this suggests to me that there might be a flaw in the ointment. Certainly the compartments in bags are much wider than those in the jackets that use the 3M featherless. That might have something to do with it.
Will also mention that once purchased a Climashielf Apex bag from Cumulus in Poland because it was considerably lighter than the offerings on REI – well under 2 lbs for a 30 degree bag. That might be your best bet for a synthetic.
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