I am a fan of synthetic gear, as it is cheap and the material science is going to catch up with mother nature rather sooner than later. I have two sleeping bags, Marmot Trestles Elite 20, and Marmot Trestles Elite 0. I like both but find the latter very bulky. I am contemplating getting a beefy liner (like,  S2S Thermolite Reactor Extreme) for the lighter bag instead of carrying the warmer one. 

Does anybody have experience with the combination of a liner + bag vs. warmer bag? How well do these options pack compared to one another. 

I have a  BD Cirque 45 for winter ski trips (it looks like it the largest ski backpack on the market), but it lacks straps for the external attachment of the gear, so every cubic inch inside counts. I am worried more about volume than the weight.