Mountain Hardwear is doing some pretty great stuff with their current line of Lamina bags. The insulation is laminated to the shell rather than stitched in via quilted baffles and such. This allows for a better warmth/weight ratio while also preventing the cold spots associated with the stitched baffles.
One thing to consider when looking at the temp rating is how much are you already going to be carrying that provides warmth. When the manufacturers rate the bag based on use in the backcountry by a civilian, its rated in a minimal clothing type scenario. A lot of weight can be saved by buying a lighter bag and keeping in mind that you can sleep in your thermal layer as well as your fleece or other insulating piece you might already be carrying. I'm sure that way out in the boondocks, you often don't want to or have the time to take these things off anyways. Why double up on the weight if you're already wearing half your "sleeping bag." Just a thought.
Also, remember that temp ratings can be very subjective. I have worked in the outdoor industry for a number of years and if there is anything I feel I am a subject matter expert on, it is gear. Personally, from the looks of bags like the Recon, I would have to say that -4 is a very optimistic rating. Now, they might be taking into consideration the same thing I mentioned above. Many Soldiers are likely to sleep in more clothing than their civilian counterparts due to the nature of the job. I have simply never seen a synthetic fill bag rated at 0 degrees or better that packs that small. My 800 fill down bag that is also rated to zero doesn't pack down that small and down is multiple times more compressible than any synthetic.
A bivy is a great idea. Consider whether or not you want one with a pole at the head. These can be pretty nice on long days in crappy weather. And some people just don't do well waking up with the bivy laying across their face. The hood also allows for some ventilation so you can stay dry and still expel your hot air. If you are down in your bag or your bivy breathing, you are only going to be colder rather than warmer. Once that moisture cools inside the bag its no bueno. One last thing to remember with the bivy is that it too will add 10-15 degrees of warmth to the bag.
Hope some of that is helpful.
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For Americans war is almost all of the time a nuisance, and military skill is a luxury like Mah-Jongg. But when the issue is brought home to them, war becomes as important, for the necessary period, as business or sport. And it is hard to decide which is likely to be the more ominous for the [terrorists] -- an American decision that this is sport, or that it is business.
-D. W. Brogan, The American Character
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