yeah, I'd like to see some sort of data on the relative warmth of clothing vs. a bag, too. As you point out, the nice thing about close-fitting clothing is that it mostly eliminates heat loss from convection in the space between the insulation and your skin.
But there is another side of that coin, too: where I can press skin to skin (like arms against torso, legs against one another), I'm in effect getting (nearly) 100% efficient insulation. Since the surfaces are (pretty close to) the same temperature, they're in thermodynamic equilibrium i.e., I'm not losing any heat at all there. To put it another way, I'm decreasing the surface area available for me to lose heat (sleeping in the fetal position would be even more efficient, but not so comfy).
Any insulation between parts of my body, then (inner arms, legs)is at best wasted: it's insulating me from myself. In fact, it's even worse, because even good insulation allows a certain a amount of convection within it, which skin-to-skin contact wouldn't.
It's tough to say how significant this effect is, but I've often heard old-timers say that sleepin' nekkid is your warmest option. I always discounted it before, but now I wonder. Counting the inner half of each arm, the inner half of each leg, and a bit of side-torso for good measure, and using the "rule of nines" (http://www.emedicinehealth.com/burn_percentage_in_adults_rule_of_nines/article_em.htm ) to figure surface area, we're talking about nearly a third of your area.
So maybe that's another reason (beside duplicated weight of shell material) to go bag-heavy, clothing-light.