Depends on the stuff sack, but I would still use a garbage bag liner cinched tight inside it. Having gone lighter/smaller , I'm much happier always having my bag inside the pack dry and protected. It is your last defense against cold/bad nights sleep... I generally use a pack cover or a tarp/poncho.
I've also found with less volume in the pack taken up (except when using a bear can) I don't have to compress the bag as tightly, so it recuperates faster and is warmer quicker. I always wrapped mine in a closed cell pad , then strapped both on to my pack in the old days. Outboard is OK, just make sure it's waterproof, and relatively rip resistant.