"Very good question I was just thinking about this today. I think it may be a good balance I was thinking a layer of 2.5 ounce climashield as the outer layer and 6 ounces of down on the inside. Pros I can see is 1 any vapor that condenses would condense in the synthetic. 2 some moisture could get in and the synthetic may stop it. 3 you still have some insulation if it gets wet. 4 smuggle cheaper$"
I really don't think you want good down attempting to support much of anything. Even large amounts of more down will cause some compression and loss of loft. For stiffer stuff, say 600fill and below, this *might* work. For 800fill and above, it probably won't. Loft, and the interlocking feathers is what what makes down a good insulator, many say the best depending on conditions. Measuring 1" of loft is fine, but it is not additive. Like air pressure, the more you put on top, the more the bottom is compressed. An example: If I measure out exactly 1" of 800 fill down and place it on top of another layer of exactly 1" of down, it will only measure 1.75" when I am done. The insulating effect might be around the same as 1.8" of down, because the interlocking is largly uneffected at this low weight. Synthetic over down will likely cause good down to compress even more, loosing a good proportion of it's loft. You will still be warm, but loose a lot of the loft from down that could make it warmer. I might suggest going the other way, down over the Primaloft to allow maximum lofting. The breathing of down is better, too.
Down is fairly immune to dampness, as long as it isn't outright, wet. It is really no worse, or better than synthetic. A plume will have it's natural curvature, often helped along by static electricity charges. The static *can* be dependent on the moisture. So, loss of loft due to moisture is true to some degree, but really, it is not that bad, and, lofting can change on a daily basis. It is also naturally hydrophobic. "Water off a ducks back" has real meaning. Each plume incorporates a lanolin like oil into it's structure. This is why strong soaps and detergents will destroy down, it washes these natural oils out of the structure, leaving the plume brittle and subject to breakage and "dusting," and, soaking up water. The shape of a plume is easily restored by a heat then drying. They use steam heat at the factories to "sterilize" down before use. I regularly steam fishing flies to restore them. Heat and moisture does not hurt down. *But*, on a bag or other synthetic shelled garmet, you can melt the plastics, use extreme caution. Between the static and the propensity of down to remain unclumped(due to the static), it is important that it be kept very clean and undamaged for maximum loft. Knowing the limits of your down stuff will put you in a much better position for dealing with it.
Compressing down does *not* destroy it if it is dry. A "static" test will often help me determine if I need to launder my bags. If I do not get a proper ammount of static after a low heat drying(usually done after every trip, before I store them,) it probably needs laundering. Shaking it out well in use, will also help me determine if they are dirty, though, relative humidity can also effect static.
The fact that down seems to be colder than synthetics when wet is mostly due to the ammount of fill they use. If you take 1/2" of wet down and 1/2" of wet synthetic material, I am betting that they are roughly equivalent in insulating power, perhaps down may be better. Never tried it, though. But dry, a 1/2" of wet down will loft to maybe 3", wet synthetic will loft to only 1". Soo, proportionally, you loose more insulating value. But, in 40F weather, carrying a 40F synthetic bag or a 10F down bag kind'a defeats the purpose of buying the down in the first place.
Down, at least everything I have purchased so far, has a long, long life time. I was reading a couple years ago where some nice feathers were found in an old grave over 4000 years old in Peru. Never store it wet or damp. I would also suggest hanging, or laying an item, as Bob says. It will easily last a single lifetime. Some of the shell material is another story. . . Anyway, looking at well maintained down gear, you find that it is generally far cheaper per year than synthetics. For my kids, I bought four sets of cheaper synthetic bags over the course of 12 years. Between bedwetting, growth, etc, it didn't make sense to buy down. At about 17 years of age we got them down bags. They still have and use them about 16 years later. Still in good shape and perfectly usable(though one was torn and repaired.) The initial price was about 5 times of a synthetic. But, they are still going for roughly the same cost per year. The cost per year will reduce as long as they maintain their down well. Which would you call cheaper?