I guess a Mountain Shirt would add a lot of warmth to a sleeping system. Buffalo make sleeping bags using the same fibre/pile. A friend of mine uses a Mountain Shirt with a synthetic bag.
If you go down the Buffalo route for clothing, then pack size shouldn't be an issue. It is designed to be worn all day, not carried.
If you are looking for a hood for the shirt, there is the standard hood, and also an expedition hood. The expedition hood is larger, it can just fit over a climbing helmet, and it has a wired peak.