Please do not forget that relative humidity has an equal part to play with heat retention, when it comes to overall "comfort". This is what our psychometric charts have been offering us for over a century now. If our bodies are warm enough, but we are in an environment where we are too humid, we won't be comfortable, and will most likely not fall sleep.
It seems that RH is simply neglected in these quantitative measurements. I'd hate to see conclusions being formed based on half of a picture. If IR barriers actually worked a practical scale, all our cars, buildings, and homes would be shiny now. The reality is that IR barriers are more effective at reducing solar heat gain than preserving human generated warmth.
Fwiw, there were sleeping bags made with high emissive fabrics back in the 80's (Kelty comes to mind.) They are gone now because they ignored the importance of vapor permeability. Furthermore, you will find less building codes requiring vapor barriers, simply due to the fact that building science has now concluded that they cause more harm than good to our environments.
If you want to stay warmer in a sleeping bag, have an overbag, make sure you have a good conduction barrier (ground pad), a good draft coller, and don't toss and turn. The sudden air exchange in a bag will render any heat retention pretty much eliminated.