have not tested this but I think the hub's friction is a small contribution to the overall drag and resistance encountered by a bike. Speculatioin is that radius of the wheel (hub to ground) moots all but the most extreme cases of high rolling resistance. I could imagine a test that measures the temperature of the bearing - comparing the best possible hub in perfect adjustment with the best grease - to a worst case bearing. See if there are any friction losses that can be discerned with temperature. Portlandjim might have done an experiment like this in the Specialized lab, they had a rolling road test rig I believe.

In order I think the sources of drag and resistance on a bike are

Aerodynamic drag
Tire rolling resistance
and driveline losses

also note that aerodynamic losses become a bigger part of the overall drag budget as the speed increases. Draq increases as the square of speed, and the power required to overcome it rises as the cube.

/markp