Your point about Northern Dancer achieving what he did as a May 27th foal explains some of his impact on the breed. As great as Horaito Luro was, the horse had to have a genetic component that was superior and passed on to his many sons and daughters.
Tapit won the Wood and then was trounced in the Kentucky Derby and Pennsylvania Derby. Somehow, little by little, he became the sire he is will be the preservation of the A.p. Indy male line as well as Bold Ruler. And he accomplished this without being that much of a turf sire.
Dick