FWIW, I think it's Youngblood who has lots to say about the vapor barrier value of a SB. His main point is that it creates a vapor barrier away from the bottom of you hammock, ideally outboard of some kind of pad slung beneath the hammock (as is the case with HH Super Shelter). With the HH, the second layer of silnylon is not breathable anyway (unlike a down underquilt), so use the SB as a vapor barrier rather than some kind of heat reflector.
I use one with my HH Super shelter and find I get "about" a 10 *F benefit. I also sling them under my two son's HH Scout models and tie the square ends up over the ends of the hammock so the SB fits like a diaper.
From other members I understand to make sure there are no air gaps between the SB and the hammock, otherwise you lose alot of the benefit.