I have a bag liner that claims to add 15 degrees to the "range" of a sleeping bag. Is it reasonable to assume that it would work the same for all bags? For instance, would it extend a 40 degree bag to 25, and a 0 degree bag to -15?
It's not reasonable to assume that it would add 15 degrees to all bags. Every bag is different, and the ground (snow? type of pad insulation?) and amount of air flow makes a difference.
Incidentally, some bag liners are waterproof and not breathable. This can cause condensation and make it feel colder inside than it would without the liner.
I'd say yes, it would add roughly the same temperature boost to all bags. This is of course assuming other conditions are the same, like the surface you are on, stillness of the air, etc.
Short of the material actually changing physically (like down getting more fluffy), one piece of insulating material will have the same temperture drop accross it for the same heat power going thru it, regardless of the absolute temperature. Put another way, if something exhibits a 10° drop when you put 50 watts thru it, it will exhibit that same 10° drop whether that is from 10 to 20 degrees or 50 to 60 degrees. Again, this is assuming the material is not changed by the temperature difference, like it melts, clumps, gets more fluffy, etc.
Of course things will vary, but as a first rough approximation I'd say the degrees added "warmth" will be about the same accross different bags inside.