I'm not sure I necessarily agree that the plywood will be "more solid" than drywall. More rigid, perhaps. Denser? More reflective? More absorbent? Don't know. Certainly different relative to acoustic properties.
Listen to bridgman. He's an effing genius, but we try not to admit it too often. And he's far too modest.
My understanding is that the only benefit of manufacturing a room "out of square" is to tame the room nodes; it will not give you low frequency absorbtion like bass traps would. And it could give you some really wonky reflections at other frequencies.
Not all "non-hard-surface" treatments are created equal. I think your scenario sounds GREAT, but fiberglass panels and foam panels are certainly going to be different than carpet acoustically.
