I've never used one, but everything I have read online is pretty dubious about those claims. They say at best you can add 5-10 deg. unless the liner is a vapor barrier layer. You can get quite a bit of additional heat retention with a vapor barrier layer, but they aren't very comfortable and there is an art to using them.

Think about how much down it would take to add 40° to a bag. If a bag manufacturer could get rating with a thin layer of cloth instead of expensive down, they would do it in a heart beat.