Larger, Breathable Stuff Sack for Down Bag?

Not that it will make an earth-shattering difference either way, but…

I too used to use a stuff sack with my down sleeping bag. I later experimented with packing the bag directly into my backpack — and found out what many others have before me — darn, it was actually so much easier, quicker and MORE space efficient leaving out the stuff sack entirely! Basically, instead of you wrestling with the bag into its stuff sack to make it smaller — the bag automatically "self compresses" as you pack all the other contents!

I line my backpack with a contractors bag for rain protection — but in your case where you are confident that rain won't reach in — then there's no reason to use a large, breathable sack either. Just pack the bag in first, and then pack everything else as normal. Give it a try.