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I got a liner last month when I was in France after a few nights of waking up at 4am cold, slept through the night with the liner, so I'd say it definitely does do something for me at least.
spotter-
Hmm perhaps our liners are different. Mine packs up into a pouch the size of an egg; the fabric's so thin I think the difference is negligible compared to half a dozen other factors e.g.
A few degrees warmer air temperature
A sunnier day so more latent heat in ground
The ground drying out
Less wind
Eating a fattier dinner (a big chunk of salami or one-serve pack of butter will keep you warm all night for very little packing weight/size)
Sunburn
Getting used to sleeping outside over the duration of a tripSorry, I try not to get too geeky about these things, but they do all make a difference in my experience.
If you want a smaller pack size have you tried a down quilt instead of a bag? I have an Alpkit Cloud Cover that has kept me warm enough on a couple of snowy butter-fuelled nights. Bit late for this trip but you are welcome to try it, in exchange for a bit of reflectix.
lae
A silk liner will add fuck all, seriously.
For thermals the absolute best are polypropylene but they don't seem to be sold in the UK bizarrely. Helly Hansen do polypropylene base layers but they are quite thin, which might be perfect for small pack size. In terms of warmth, weight, and pack size, merino and polyester are about the same, in my opinion.
3mm closed-cell foam under yer puffy mat will add a lot of warmth, if you can find a sensible way of carrying it (assume you are on a bike).