Yeah, I tend to think that has a lot to do with it too, Mark.
For many years I used silk long johs as a base layer while fishing in below zero temps (Steelhead bite pretty well in December, January and February following a salmon run.) Often, temps were 0F and below at first light. As part of a layering system, they really excell at low volume, high heat retention. After the silk wore out, I switched to modified pantihose. These worked pretty well, too.
Again, I really don't *think* they are any cooler, they just cause perspiration to accumulate outside of them. You think "cooler" when your skin is dry, whether it actually cooler or not...perception.
BTW: Here is what worked for me, silk, light weight nylon/poly(mid size), heavy weight wool(large size), synthetic quilted of some sort(large size), oversized pants... About a half inch or so when all was added together. Waders were also oversized, requiring a seperate set for winter water. Worked for up to 10 hours of wading and fishing. NO DOWN. The waders held every bit of sweat.