That fluffy insulation will settle after while in an attic. Imagine how it will settle when you shake it for a few hundred miles. I think it would absorb moisture over time and flatten even more.
A pro might be able to spray icynene foam up there. Not certain. Actually, polyurethane would be a better foam since it's closed-cell and can't absorb water (icynene is open-cell), but I suspect the polyurethane expansion would risk breaking the ceiling.
I personally don't think the air space provides as much "thermal break" value as insulated space would.
