If warmth-to-weight is your focus, I'd skip the liner in favor of the equivalent weight of additional down in your bag or quilt.

I personally find liners to be a PITA, only use them in hostels and similar situations where they're required. But I know others who really like them, so I think this is yet another area where you're best off establishing your own preference. Perhaps you can borrow one from a friend, promising to gently wash it before returning it? The warmth-to-weight ratio isn't THAT unbalanced, so it's always possible that you'll find that it works for you. One other benefit that liner fans mention is that in warmer weather, sometimes the bag is a bit warm and having the bag as a sort of blanket plus the liner gives them some temperature flexibility.
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Brian Lewis
http://postholer.com/brianle