I'm not a physicist but wouldn't this reflective effect be tiny compared to the prevention of convective heat loss?
The reflective action is impressive. They also act as a wind break and turn whatever clothes you are wearing into a more effective measure against convective heat loss.
The poor thermal insulation and condensation were predictable. I hadn't predicted the shifting wind which chose to balloon my orange cocoon with cold air every 20 mins no matter which orientation I chose to lie in. I finished the night with a bungie cord around the sack, under my armpits. That of course left my head, shoulders and arms exposed but kept the wind out. Not recommended.
After this experience I upgraded to reflective mylar survival bags. NB "bag". These are a little harder to find than survival blankets but if a bag can be so badly affected by moderate wind, a blanket would be so much worse.
The range is a low of about 50--75W (1,000--1,500 kcal, effectively starvation levels), and a maximum ceiling of around 485W (10,000 kcal day, perhaps a elite endurance athlete). Well below 1 kW in all cases.