"I would put my down bag in a dry bag, roll closed smashing as much as possible."
The trash compactor liner bag approach IS the most bombproof in terms of water protection.
But I don't use it. (Go figure.) But I do want my 15°sleep bag and camp gear dry no matter what. So like Jennifer (did), I stuff my bag etc into a Large stuff sack, but one with and eVent bottom, which then goes into my pack. In fact, it completely fills my pack. Until I lean on it. Then the air squishes out through the eVent, and then I turn it sideways. Then I layer in the rest of my stuff. The eventual volume of the sleep gear dry bag becomes Very small, and more importantly, is amorphic, filling every possible space in the bottom and accepting the intrusion of everything on top. (There is no "beach ball effect".) It virtually flows (under pressure) into and around everything. A Large bag permits a Lot of flow and go, a Small would be self-defeating.
This "fluffy stuff" management is what I like and why I don't go with the trash compactor bag
stuff-it-all-in approach. FYI, I use a ULA Ohm (40 L) for no-canister 12-day hikes.
You can find the stuff sack here: eVent Drysack