I have a Never Summer Heritage X which has the type of rocker you ask about. It really does make a big difference in powder. It's a 163 and in powder (your specific question), it destroys my 161 Donek Wide and rides about as well in the center holes as my 169 O-Sin 3800 does in the rear sets. Of course the Donek destroys it carving (especially firm groom)... but it will carve ok in forgiving conditions. My only knock is that it feels a bit squirrely riding it flat... can be a bit disconcerting, but I never had it catch or come around - just doesn't feel like it's tracking like a cambered board.
I think the rocker makes it faster and easier to maneuver and more forgiving in bumps and trees too - definitely the quickest and most fun board I've ever owned in that regard... again, destroying the Donek in those conditions, and quite a bit quicker than the bigger osin.