The plasma has gates within the vertical baffles that stop down from migrating too far. You just aren't going to have down moving from your head to your feet since the baffles aren't as continuous as they appear.

With that said I'm very glad to hear people feel that they can rely on the EN rating system. It gives me a better idea of what I want, and I think a 15 degree bag is for me. I think I roll around too much for a quilt, and sleep too cold for a 30 degree bag.