From Roger Caffin:
"For DWR to be of any use in a quilt or a jacket, it must stop the down from collapsing. There are two different cconditions it has to handle: water, and ice.
First of, note that water or ice can still collect all around the micro-fibres of a down tuft, even if the surface of the down tuft is 100% waterproof. There is nothing to stop it. A DWR-treated shell (or even a GoreTex shell) cannot stop the water vapour from getting inside the shell either. Only a vapour barrier can do that.
Ice itself around the down tufts inside a shell will cause some problems, and no amount of DWR on the down will do anything about ice forming inside your quilt if it is cold enough.
What about dampness (rather than ice)? It will soften the keratin in the down tufts and leave them limp and flat, with a loss of loft. To handle this you need to block water vapour from penetrating into the down tufts. But a DWR does not do that: it blocks liquid water from 'wetting' out the surface. So water vapour will go straight through the DWR layer into the down fibres and you will end up with a limp zero-loft mess.
This is why companies like WM are not convinced about these DWR treatments. They do not work in Real Life. They only work in gimmicked demos with the down stuffed in a bottle of water. Its 99.9% marketing spin again.
For me I am much more worried about water vapor when there is a high humidity level or just from me rather than rain. I wish there were better synthetics than fleece but that is mainly what I will have to use around freezing because I refuse to buy synthetics that don't have a long usage range.