The "ID" Hot Sock is made with PrimaLoft Sport. From ID's web site:
"The 4oz Primaloft Sport insulation and Pertex P685 shell fabric ensure a system that is soft, comfortable and water repellent, with 1/2" (1 cm) loft around the foot. The back panel of Powerstretch fleece makes the Hot Socks easy to slide on and keeps them snug around the feet and ankles. The footpad is reinforced with abrasion resistant 500 denier cordura, which is suitable for use inside huts and tents, but is still soft enough for use as a sleeping sock."
With a 1/2" loft around the ........, They say they are using the 4 ounce per sq yard PrimaLoft Sport insulation. Doing a little math with the Clo for this product I came up with 2.96 which is (more or less) same as down loft. That amount of Down should be good to below freezing I think. If they warm your feet quick your feet might sweat a little and make the feel cooler.
The PrimaLoft Footwear stuff in the thread is rated by "R" value and goes from 0.20" (R=0.27) to 0.30" (R=0.41) to 0.47" (R=0.64). I haven't used any of it yet so I don't know how warm etc it might be. It is made for insulated footwear.
I am going to make a pair of insulated booties out of three different insulations and the "Footwear" stuff I have to compare them to each other. I am working on two packs at this time so I am not sure when I will get the booties made.