I've just noted the new range at www.kimmlite.com, and there is some really interesting stuff (Note: I haven't seen any of the new gear and it doesn't seem to be in the shops yet).
For the majority of you who are North American(!), Kimmlite is an established UK brand (with which I have no affiliation, BTW). From a UK perspective, Mike Parsons of Kimmlite is regarded as the father of mountain marathons AKA 'adventure races', and spawned much of the equipment for them - most of it is relevant to lightweight backpackers!
In particular, note the PA1 sleeping bag - a 360g (> 13 oz) bag with NO insulation in the upper body, but a full mummy shell!
The idea is that you wear a duvet jacket (sorry, = 'down parka'!) or a synthetic insulated jacket, such as their own 'Pumasillo P' inside.
This is an interesting take on the old 'pied d'elephant' (='half bag'), in that it helps you keep your arms in to your chest, where they remain warm as well as providing draft-proofing.
Intuitively, this seems 'right' to me. The half bag is one of the oldest weight saving ideas out there (it was mentioned in my climbing reference book when I was a kid in the mid-seventies!) but when I've used one, I've always found it difficult to know what to do with my hands (OK, get the cheap jokes out of the way now!). If you can put them inside your jacket then the jacket is too big to be affective, and I for one can not get comfortable with then tucked inside the top of the bag.