Thanks for everyone's input. I've ordered a thermarest xtherm just to see what they feel and look like. So expensive! I have a "spend nothing" alternative, using stuff I already have: thermarest venture mat + old ccf mat + wool blanket. There's a massive weight and size difference when packed between the two options: 430g vs 1.9kg and 21x10cm vs 49 x 22cm. I'll test drive the spend-nothing option tonight to see if a beefed up mat makes the warmth difference I'm looking for.
Multimat make excellent quality SIMS for a lot less than Thermarest?