you are correct to say that down is reliant on it's ability to loft; but then so is most other insulation that isn't radiation barrier or aerogel!!!
The down jackets that I have owned have been sized to go over other layers. I usually have a waterproof that is big enough to go over the down jacket, but rarely need to wear the like that. If it's cold enough for the thick down jacket then it's usually too cold for liquid water. The waterproof shell is light enough that it really doesn't compress the down very much.
However, many of the new style, lightweight down jackets are sold as potential midlayers. If it was big enough you could probably throw on a synthetic belay jacket over a down vest or light jacket. It would compress a bit, but not so much as to render it useless.
On a tangent, I've read a lot of US military literature about their cold weather clothing systems. They use fleece & synthetic wadding layers topped off with a down filled parka to make up their Extreme Cold Weather Expedition Suit (ECWE suits). Their data gives the Clo value (a US standard similar to the Tog where 1 tog = 0.645 clo) for each item individually AND then the clo value of the ensemble which is always less than the sum of individual clo values (by about 20-30% IIRC). That is to say that in their lab testing, adding layers does add warmth but not the warmth of that layer on it's own as it inevitable crushes the inner layers somewhat. For that reason, I would always advocate wearing the down on the outside where possible. The only caveat is that I sometimes add a light synthetic sleeping bag over my down bag but this is to keep off damp & ensure the dew point is outside the down. If that kind of stuff interests you, then have sift through some of these...
If you are concerned about having a down layer that can also cope with changeable wet/cold conditions then I'd point you towards the Crux Plasma as it uses a very lofty down under an eVent shell so there's no additional compression & the breathability is as good as it gets right now using a membrane.
If you are concerned about having a down layer that can also cope with changeable wet/cold conditions
John - in my case not concerned, I have years of experience of coping with damp. I did use to pack an extra mid-layer and cower behind a wall to remove out waterproof, add mid, re-fit outer. I then stumbled on the overlayer which works great, and because your waterproof is damp, a synthetic overlayer is ok. Last winter in UK, I begin with wearing mid-layer and use vents during climb, de-vent, then overlayer.
In this thread it's simply minimum number of layers to cope with a rapid 20C drop in temperatures to sub-zero, damp not the concern, apart from snow melt when coming in-doors and I'd brush that off as much as possible. As such garments are high cost, flexibility to use in diverse situations is of great interest. How many garments end up just used a few times and then replaced with something better... you all know what I mean. . I bought my Rab Drillium, for example, and warm+wet hardly arisen for me.
My local outdoors store is REI, they have some good stuff, many brands are different.
> They use fleece & synthetic wadding layers topped off with a down filled parka
Interesting; that's not the order I'd recommend for a mixed-fill sleeping bag. You want to keep the down next to the body to keep the dew point out of the down, and in the synthetic, to prevent condensation in the down.
Of course, the down overjacket may have a vapour barrier liner.
Ah... but this is a clothing system not a sleeping bag. Usually they don't use down as far as I can tell except for static sentry type stuff - and having it outermost makes it easier to take off when they start doing anything active. They work out all their activities in METS and then use a table to recommend how much clothing is needed at a given work output in a given temperature.
Ordered a Patagonia Down Sweater Vest, $110 delivered (approx 75ukp at current exchange rate). Is 800 European down, with water-resistant outer.
Expect to have it in my hands in around a week. For my future winter travelling outfit I would have a regular baselayer (to cope with regular indoors), a Paramo Trekker hoodie (reversible, cool-side for regular indoors, warm-side for cooler), a Montane Featherlite for shower-resistance, and (now) the Patagonia Down Sweater. Note that all my layers above base are full-length zip for easier removal.
I expect to warm enough to around -10C, and below that be dependent on fast walking to the next indoors!
This cannot get me to the -15C that a large down jacket would provide, but in limited luggage has a good warmth/volume ratio. If I do have to encounter more serious cold, will consider adding a down jacket over the top. Was tempted by the PHD stuff....
Seems practically any time of year other than December gives best price for down , looking at OM's past posts, can get 25% lower cost between Feb-Sept , so that would be a better time to get a down jacket for the more serious cold.
Nigel, I'd say you got a good price there! And it's a good product from all I have been able to tell. As you say, may not be enough for harsh cold but will pack very small and give your existing kit a decent boost in colder weather.
John, also, I noticed in your past posts you claimed Patagonia use 800 USA down filler, but they claim to use 800 European down filler, but that could my incorrect inference. See your post and compare to Patagonia's "800-fill-power premium European goose down". So is Patagonia using the Lorch standard, or the USA fill-power measure, but sourced from Europe, which would mean they use the USA tests on European down, and consequently not as warm?
It's pretty hard to know what measurements different companies are using, but now that you mention it, I think I have a Patagonia catalogue in French that claims 800fp so it's likely to be the EU measurement. Either way, I've have a close up look at these jackets and they are typical of Patagonia's approach and, as far as I remember, they were the first (maybe along with Mont Bell) to bring these lighter down products to the wider market. My comments on another thread were possible comparing them with the looky-liky new jackets from Rab that I accused of being bandwagon products as they don't use Rab's top fillpower down. My wife's TNF Catalyst is pretty similar to the Patagonia in terms of weight & loft and it claims to be 850 US fp, so I think that correlates. The Patagonia seems to have the edge on manufacture quality too...
Note I went for the sweater vest, it's thinner than the regular vest. There is a negative criticism on it's wear capabilities here "Unfortunately it is ridiculously fragile, seems to tear on everything, and the down leaks out like water once it has a hole. Wishing I had bought the Vest, rather than the sweater vest..".
In my context, it's going to spend a lot of time stuffed for the colder days, don't intend to wear as an outer with backpack, but also down is really meant for belay with backpack off, or campwear, sitting around, etc. Not really a walking+backpacking context (for which we got the shells & P&P type stuff!).
So I get home from Tampa (81F) to home in San Francisco (52F), to find already delivered my new Patagonia Down Sweater Vest.
It is NOT as warm as I would hope, it's slightly less warm than the Paramo Torres Gilet I was hoping to improve upon, partly because it is more cut away behind the arms and not as high around the neck.
It is small+light though! It will take up less room than my Torres, which is good as I will be packing gloves etc in the new year. I would agree the 800 fill down has good warmth/weight ratio but was hoping for more warmth. Why are these garments so thin? I was consciously going for a vest for mail-order as with vests the cut is not as critical so less risk buying "blind", if I do also get a down jacket, will ideally like to try it on first.....
. I bought my Rab Drillium, for example, and warm+wet hardly arisen for me. 
, looking at OM's past posts, can get 25% lower cost between Feb-Sept 
