> the data showed that on summit night, the average body temperature difference between the twin in modern down and the twin in complicated layers of silk, wool, and gabardine was a staggering 1.8°C. > “In a hundred years, you’ve gained—arguably—one degree of efficiency per 50 years,” Ross reveals.

Depending on where the baseline is, 1.8 degrees could be huge! But more importantly, heat dissapation is a non-linear function. The warmer you are relative to your environment, the more energy is lost. While Shackleton's kit forms a lower baseline, it probably makes sense to imagine how some imaginary perfect vacuum insulated sleeping bag would perform.