Thanks, Alf, I appreciate your suggestions.

The Vango venom 200, BTW, is rated at +5C ("suggested minimum usage"). That's 41F on this side of the pond. The -9 degree rating is the "extreme" rating, that is, it would keep you alive at that temperature, but that's about it. It's not comparable to either the Western Mountaineering Ultralight or Summerlight. I have both those bags (30oz and 20oz) and have never wished I'd "chosen more wisely." They were quite a bit cheaper when I bought them years ago, and of course cheaper in the US, but still expensive. I hike mostly in the Oregon and Washington Cascades, and I get cold fairly easily, so the Ultralight is usually the one I take.

I looked at the Klymit Inertia X-Frame online. Interesting idea, but looks uncomfortable for a side sleeper. The Thermarest Prolite4, BTW takes zero breaths.
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Always remember that you are absolutely unique, just like everybody else. -Margaret Mead