Over stuffing down garments actually produces clothing that is not as efficient warmth for weight. The optimum amount of down is that which can loft fully without being too loose. Once the down cannot loft fully there is no advantage in adding extra down.
I understand that this is the generally accepted view. But Richard Nilsey has some very persuasive data which shows otherwise. In particular, it appears that down can be stuffed at about 2.5x the density that it is normally done, whilst still getting full value for warmth. The idea that down needs to "loft fully" so that it spreads out as much as possible is not correct (again, according to Richard's data and some published papers). Having said this, there is a point at which adding extra down starts to produce smaller gains, and eventually you even go backwards. But there is a linear region where warmth increases proportionally to the amount of down (even if it is "overstuffed").
I don't have the links to the threads where this was all discussed, but if you have a search through Richard's posts in the past year you will find them. Have a read, it is very enlightening!