Another thing to ponder...
Van Dyke, in The Still Hunter, advocates using a bead front sight and a flat rear sight with no notch, suggesting that the eye will automatically center the front sight in the middle of the rear bar. While not directly applicable to the 18th and most of the 19th century (he was writing in the 1880s), his ideas did make me start wondering if our ancestors may have used a quite different sight picture then we use today, using the rear notch for horizontal alinement only and raising the front sight above the plane of the rear sight ("fine bead, coarse bead") rather than keeping it level as is the practice today. I know some folks think that the buckhorn rear sights may have been used like a modern aperture sight at times, which would be similar to what I am suggesting. I don't have a gun with low sights to test my theory, but I suspect that used this way, those original sights would be a lot easier to use.
Bob Spencer, over at another forum, suggested that the low original front sights may have acted like a bead, so I think he may be thinking along the same lines.