Those Performance tights as posted by LITW look excellent. I was lucky to get my PI Amfibs cheap at Sierra Trading Post. Otherwise I'd go with those.
But why, I ask you, doesn't anybody make winter *bibs* with all that good windpanel construction? Even with a longtail jacket, the small of my back sometimes gets cold, and bibs give you extra protection in the chest, too. Yes, it's more hassle to "answer Nature," but that's true of lightweight bibs, too.
PS: my sources tell me that Performance, in addition to owning Nashbar, has picked up Supergo as well.