Interesting what WM has to say about treated down......
I had a WM Antelope GTX for over 20 years. The shell was regular Gore-Tex back then, not the Gore Tex later developed for sleeping bags. A bit stiffer, more robust shell, and didn't pack down quite as small. Bought it in 1990 and a month later spent 15 days on the river in Alaska on a DIY moose and caribou hunt. We were blessed with better than average weather, but had a 5 day, 4 night stretch of really wet weather where it rained most of the time (as opposed to just drizzle) and two days of 15-30 mph winds. By the fourth night the down had begun to wet out, lose loft and warnth. The following year backpack archery hunting for elk in the San Juans we hunted in a stretch of wet rain/sleet/hail/snow for 4 days, and I could tell the bag was losing it's insulation ability again. When I went back to Alaska in 1993 for a repeat performance, I brought a North Face synthetic bag--and we had more days of rain on that trip. But the bag did fine. Of course it weighed more and didn't pack down as small, nor was it rated for as cold of temps, but on a raft in Sept in Alaska, it was a good choice for the conditions.
As I eventually figured out--and talking to others--the down wasn't getting wet from the outside in, it was becoming damp from the inside out. After hunting in rain, no matter how militant a guy is, it's impossible not to drag water into the tent, and with extremely high humidity conditions, plus your body is clammy because even with the best rain gear a guy gets.....well, damp. One of the disadvantages to any water resistant, breathable fabric is as body vapor, plus any water on us, can quickly condense into water as it slowly makes its way through insulation and also encounters the water resistant-breathable membrane. I loved the Antelope for it's light weight and warmth in the summer at 12K, or while winter camping while teleskiing, (15 below outside the tent one January at 11.5k ft) but I stopped using it in September when there was a good chance to encounter wet, coolish conditions, when things would not have a chance to dry out. Plus it takes down a long time to dry.
Alas, the Antelope was about right for me in my 30's, but by the time I gained another 10 lbs in my 50's, it was too small--I plumb outgrew it
I sold it about 5 years ago (WM bags are like Swaro, they hold their value).
These days my neck of the woods are well populated by millennials and Generation X'ers who are adventure travelers of the first order. Lots of expedition climbers, and backcountry skiers, etc. The word from them was buy the treated down. It allegedly wets out a bit slower and dries noticeably faster. So I bought a KUIU 0 degree bag when they had their Black Friday sale a couple years ago. Although I've used the bag, I have not really been in the kind of weather to make a determination as to how effective it will be in extremely wet conditions for days.
I do like the bag so far. It is lighter than my Antelope, stuffs a bit smaller and the 0 degree rating seems to be reasonable.
