Just now read your post.
Lots of use out on the coast and the Strait in heavy rain and damp cold. Some mountain use in light rain, but no snow conditions yet. Dry, yes. Warm, not really.
The biggest difference between your camper shell and a FP is going to be increased usable space and a comfy mattress when the tent is deployed/flipped.
I carry rain gear, use warm clothing layers, have a -5degree bag, and almost always use the rainfly.
Have you had the chance to see one of these (ATFP) up close?
If not, let me know, and we'll arrange to meet if you'd like a demo.
Walt