Tom,
I switched from a vacuum advance to a purely mechanical and except for the initial timing advance setting there doesn't seem to be any real difference that I can attribute to the distributor itself. I must admit, however, that I am handicapped by a pretty insensitive seat of the pants. Of course, their were also some other factors involved. 1) They were both used distributorsn and, although they were both in good condition, who knows what differences they had developed over the years. 2) I installed a Crane flasher unit at the same time, so I'm inclined to attribute the performance improvement I do notice to that.
Bob S.