I'm not a naval architect, but 'beaminess' doesn't help with waterline. The waterline length parallel to the centerline is the important parameter, which is why heeling boats with overhanging sterns increases the length. Beam adds to the waterline only by adding a 'curve' to it. The waterline is important because it determines where the bow and stern waves set up, and how the boat 'surfs' in its own wake. My Ranger 33 does increase waterline a bit with heel (due to the transom shape), but only slightly. The "flat is faster" rule is more for dinghies and non-displacement boats (modern sportboats), but once you get beyond 15deg of heel in a 'classic' hull shape, you start slowing down.
I race in pretty nasty and variable conditions, and if I'm caught with a large headsail up with no one brave enough to venture onto the foredeck, I pinch like crazy to reduce heel if going to weather, and let things twist off down. Once you're heeled beyond 15deg, you can usually reduce sail and go faster.
