No heat James, just more about my technique.
Instead of packing the weight of down for the underside of my body, I would rather carry a slightly thicker pad to boost the R-value of my sleep system. If you are a restless sleeper I'm sure the down in the bottom of you sleeping bag will serve you well when you're tossing and turning. This is not a concern for me, I'm a back sleeper.
I also bring an insulated jacket with me on every trip. If it is cold enough, I will wear it to bed. The jacket will fill most of the nooks and crannies, and doesn't add to the weight of my pack.
Back to the question at hand. Ben, there appears to be no answer for you in the near future. There are too many views and even more types of sleeping bags and methods of manufacturing. The other problem is how you camp(tent, tarp, hammock, open air) and what type of ground insulation you use(or more importantly the R-value). All are factors in how warm you sleep in any given bag. Until there is a standard in how all of us camp, their can't be a standard for how we veiw the warmth of a sleeping bag. The only standard is trial and error.