I'm not a big fan of a bag liner for warmth (might make sense to keep the bag clean, but IMO doesn't do that much relative to the weight and hassle factor for warmth).

But I do like to augment my sleeping bag rating with insulated clothing that I'm carrying (and wearing) anyway. Just under a year ago I was using a down parka inside my 20F rated sleeping bag to sleep comfortably down into the teens --- I'm completely certain that it made a big difference, given that my sleeping bag was sufficiently large that it didn't constrain the insulation of the parka.

I'm do think that if the only criteria is to have a warmer sleeping bag for X weight, then it's more efficient to put all of that weight into the sleeping bag insulation. But given that we often want some relatively warm clothing too, it only makes sense to me to factor that in when selecting the sleeping bag.