In case anyone is interested, Arc'teryx just told me that the Gossamera fabric used on the Squamish windshell has air permeability of 7 CFM.
As I mentioned before, this fabric has a light polyurethane coating on the inner face, which helps with water resistance. The fact that the fabric is air permeable means the PU coating is microporous, or discontinuous. The Arc'teryx rep said the coating was very thin and evenly applied by a fabric mill in Japan known for their coating technology. I asked which mill, and he replied, "Toray, I think."