It has been in my experience, that as a tree matures its productivity will increases. One can not accept to harvest 100+ fruit from a 2 yrs old tree, it's impossible. Given the right care (fertilizing, root pruning and etc..), along with the location of a tree (what zone it's growing in) and the type of the cultivator (if right for climate), your harvest will increase as your tree ages. As of today, the most productive trees in my collection are, LSU Purple, VdB, Conadria & Bruswick. I do have other "late-very late" ripening varieties that do produce lots of fruits but only 30% of them will ripe. They need a warmer climate. Do I consider those "productive"? No.
I hope this will help.
Navid.