I wouldn't worry about it too much, down's pretty resilient stuff. I prefer using a lightweight nylon dry bag to a compression sack. Why? Because I can get the sleeping bag compressed to the same volume, but in a mold-able shape... ie, I can get the bag tiny, then shape it into a tiny brick or something. Packs a ton easier than the hard rounded melon you get when using a compression bag!
Just fold the top of the bag once, leave the sides open, sit/kneel on the sack to force the air out. Once it's compressed, finish rolling the top down, only 4-ish turns, then flop the bag on its side and knead it out. Lots and lots of canoe tripping done this way, never a wet bag.