Definitely depends on the totality of conditions.
About 1.5 yrs ago I did the Laurel Highlands Hiking Trail in western PA and it rained a lot, and while the relative humidity (RH) was pretty high it wasn't anything like that constant socked-in fog and drizzle for Seneca/Spruce. It was late May/early June and I had no problems with down despite frequent rain.
Last fall there were some perfect conditions forecast for the development of fog so I did a local overnighter in my usual test venue, which is Harriman State Park. It was very moist and everything got damp, and my conclusion is that I still wouldn't want to be using down, DWR or not, for several straight days of this. Even with DWR there was noticeable reduction of quilt loft.
My test venue...
Moisture on tarp... no rain, just condensation falling off trees!
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