While I'm certain u would know much more about this than I,.. Is it not also true that with a stiffer cambered board, more speed is essential if u hope to get the most from the boards performance capabilities?
I'm not 100% sure, but I think the physics of it apply at any speed. The difference is you'll probably notice the effects more at high speeds when you'd be long past the limits of a softer, shorter edge, and/or more rockered board.
As far as stiffness goes you're probably right, at slower speeds (i.e. just cruising at 30 km/h or less) a stiff board might actually be a hindrance. I still wouldn't want a wet noodle for "sight seeing" but a medium flex, medium length camber board would be just right methinks.