I've used the Sea 2 Summit Thermolite Liner (http://www.seatosummit.com/products/display/24) with my Ultralamina 32 in down to -10C (~15F) and it worked OK. This was of course well beyond the limitations of the bag, or even the bag and liner combined. I think the liners should work fine by themselves, and as long as they're used within a 10 to 15F range of your bag, should do fine there as well.

Layering does in fact work, and many bring liners just because they don't like the feeling of nylon.