"actually motorcyclists are using their knee as a lean gauge and to a lesser extent to help pick the bike back up coming out of the turn. Their body position is what is used to complete the turn, not the dropping of the knee."
The motorcycle example is the opposite of Phinney's method although they both stick their knees out towards the ground. Look at the hips and the COG of the rider. The motorycle racer has all of this body on the inside and lower than the bike; thus keeping it more vertical and the tyre-patch stays flat on the ground. Phinney dumps his bike to the inside and lower than the body, making it lean more than the body's COG. I can tell you from experience that riding on the sidewalls of the tyres generates no cornering grip...
Another reason I don't like to stick out my knee on a bicycle is that it's a tremendous amount of wind-drag. The parachute effect probably has a lot to do with the feeling of stability as well. Personally, I try to stay as tucked as much as possible to preserve as much speed as possible. On the motorcycle, the power easily overcomes the drag and the emphasis is on cornering speed anyway. On my rice-rocket motorcycle, I use the knee to feel the exact angle of the bike's lean and if I'm already on the edge of the tyre and need a little more cornering speed, I'll slide down even further on the side of the bike. Randy Mamola was pretty insane with that one, the highest part of his body in the corners is his knee that's draped over the seat. The rest of his body's is about on the same level as the wheel-axles and his elbow's also grazing the ground...
"2. More weight - more potential energy, more speed in a direct tuck situation."
Well, almost. It's not density as all humans have roughly the same density. It's the weight to frontal surface-area that counts. The same rider of the same weight will downhill faster if he's in a tuck rather than sitting upright. A rider that's twice as big, will have roughly 2x the surface area facing the wind, but will weigh 8x as much. It's the much larger amount of mass pushing behind the slightly larger surface-area facing the wind that gives bigger/heavier guys an advantage on the downills.
But the tuck's still more important. Going head first, jumping out of a perfectly good airplane, I can get up to about 220mph vs. 125mph if I'm falling flat with my arms and legs spread out wide.
BTW, if any one needs visuals on the two riding positions, I can draw some pictures...