In reply to henwardian:
This has to be the most interesting gear post (to me at least) I've seen on UKC in ages. Sorry for what has turned into a mammoth reply.
The fill power (FP) multiplied by the mass (M) of down (FP x M = "insulation power") equation is actually used by quite a few manufacturers, but by none of the big ones. It tends to be used for sleeping bags more than down jackets, too. Cumulus, for example, use it.
The units of your "insulation power" value are simply volume (inches cubed (yuck)) so it's probably better to call it insulating volume, and that's what some of the manufacturers call it. Some of Backpackinglight's forums are awash with this sort of thing but unfortunately they sometimes forget other key factors that influence the THERMAL performance of a jacket:
1) the fit. This is so important and few people think about it. If you are bellowing and chimneying heat out at the hem, cuffs, and collar then you might as well not bother. Similarly, if the jacket is so tight that you compress the insulation then it is a waste of time. For me, the Arc'teryx Atom synthetic jacket is completely brilliant because it fits me perfectly. The Cerium down jackets, however, are worthless to me, because they are too narrow across my back leading to compression of the insulation and subsequent thermal bridges that will make your back cold. You simply must try on insulated jackets and if they are too loose or too tight then don't buy them.
2) the design. As other posters have mentioned, the construction techniques play a big part in how the jackets perform. Box wall or trapezoidal jackets (big dollar) are warmer than stitch-through because the insulation is more evenly-distributed, there are fewer cold spots, and the jacket is also more compression resistant. More here:
http://www.ukclimbing.com/articles/page.php?id=5163 (check out the down jacket with the infrared camera - heat pours through the stitching). Overlapping areas of fabric make a jacket warmer but openings make them cooler. Open pockets can often allow masses of heat out.
3) the face fabrics. A very windproof fabric will allow no air through which sounds good...? Maybe not. In high winds the fabric will compress the insulation and drive out the warm air. A slightly-permeable fabric may be better as it will allow a bit of air through, making the filling less liable to compress. The stiffness of the fabric also influences this.
4) other stuff. There's a lot more things too, but they are the big factors. What if the baffles surrounding the down are too big/small...? Could down be at the wrong density...?
The thermal performance, however, is not the only factor to consider with an insulating jacket: remember durability, features, fit, appearance, ethics, etc all play a part.
I think manufacturers really really should state the weight of down they put in their jackets, and most do. And I think the FP x M = "insulation power" equation is very useful to know, if you ensure you compare like-for-like with sizes (S contains less down than XL, obviously). However, it's not as simple as buying the one with the biggest number, especially when the fill powers or fill weights might be out by a fair bit in some of these products.
When buying a down jacket I'd make a longlist based on the 'insulation power' equation and which manufacturers I think make good down kit then would go to a shop and try on as many as I can, ruling out 80% because they don't fit or I don't like them based on features. That would probably leave one or two jackets and I'd probably buy the cheaper one or the bright-coloured one!
Fire-munki, they do sometimes do this sort of testing but it is very expensive. Thermal manikins often cost £100,000+ and are notoriously hard to use and easy to break. Most manufacturers simply cannot afford the testing or do not prioritise it. Tog testing is a lot simpler, cheaper, and easier but tests fabrics and down, not the features. A thermal camera (£10,000) combined with tog testing is a good halfway-house, but user-trials are every bit as important.
Post edited at 13:25