Seconding the comment above about pad insulation - In my experience, adding a bivy under an existing shelter is going to add minimal warmth (or risk condensation with the heavier bivies) but a closed cell foam pad used in combination with an air pad makes an instant difference.
I have some La Sportiva Karokorams that use a suede leather upper and no Gore-Tex for water resistant breathable combination. I love 'em in the right conditions.