BudhaSlug,
I also recently purchased the De Soto PolyPro after years of using all of their warm weather gear. I took it out 29ºF with a slight breeze a few weeks ago (before taking a vacation to Florida where I obviously didn’t need it.) At first, it felt like I could feel every bit of the cold and wind! But as my body got moving I felt very comfortable at that temperature. As mentioned above, it is possible that if it were relatively windy, you might want to add another layer, but I have not tested that yet.
I know you said it was sold out in Small, but I wanted to let you know that this is an very formfitting garment. If you own any of the De Soto Long Sleeve Skin Cooler tops in small (which I highly recommend), it is basically the same fit except for a thicker material, which can feel restricting at first. Depending on how you prefer your tops to fit, it is possible that a Medium would work for you as well.