MT,

Along those lines, here's what I use for a bag:

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The main advantage over the traditional down bag is that you don't have any insulation on the bottom of the bag, saving space and weight.  Whatever sleeping pad you use provides the lower insulation since any down on the bottom of the bag gets crushed by your body and provides virtually no insulating power anyway.

I've used this bag for several years and it's the best, lightest, warmest system I've ever used.