Assuming one can lock either wheel, and equal skill in modulating the braking force for any two brakes, the stopping distance will be entirely a function of the mu (coefficient of friction) at the contact patch between the tire and the road.
I will grant that from the hoods I can surely stop faster on modern dual pivot brakes than vintage ones, as I cannot easily either apply enough braking force with my index finger to lock the wheel, or modulate when trying. From the drops though, I'd wager there'd be little or no difference in stipping distances between any properly functioning brake capable of locking the wheel being braked all else being equal.
I agree that pads go bad with age, and this may in some measure account for people's perceptions that vintage brakes are inadequate. Who supplies pads for old Campy SP holders that are actually fresh and haven't been sitting in a parts bin for years? My brakes could really use a FRESH set.
Kurt Sperry
Bellingham WA