I didn't have any luck using space bags to store things - they reinflated right away. I have put left over batting in bags and squeezed all the air out, then stacked stuff on top of them. The poly batting fluffed up again after a few hours of sitting out in the air.
I don't know how much more air you can get out of a commercially bagged poly batting though. They take the air out to reduce floor size for shipping and sales area.