I think the difference between down and treated down is very little- if exposed to much moisture (externally or internally from sweat/breathing) it's going to lose loft; if conditions are going to be on the drier side you're not going to beat down for warmth/weight (or volume), if wetter conditions are expected I use synthetic (Climashield Apex) and take the weight/volume hit.
+1 on synthetic. Especially at temps/conditions where wet is likely.
