The differences are not just weight and finish. If you look at the cones on a high end hub, they are machined to close tolerances and mirror polished. They also use the best quality ball bearings. The net result is that they spin with less friction.

I ride a number of old Raleigh bikes. Super Course/Competition GS/Professional. Some are equipped with Maillard/Normandy. Others Campy NR and Gran Sport. While hardly a scientific study, I've noticed I can easily add 1-2 mph on my average speed on a Campy equipped bike on my regular 40 mile training ride.

My spouse rides a Normandy equipped Raleigh and she hates it when I'm riding with her on Campy hubs because I spend most of my time coasting while she is working.