You must need to get really small.
Sorry couldn't help it.
It probably shouldn't, but just the same, I wouldn't. But I'm packing and you're stowing. Different strokes and all.
Usually down stuffs pretty well, and a large contingent of LW and UL hikers are going with no stuff sack, as insulation seems to fill the dead spaces in your pack better that way.
Some will no doubt use compression with Syn but that's apples and oranges again too. If you do decide to do it, take pics. You're probably talking hardball size down from softball, and nearly as hard