But a prayer to God directly must be more efficacious than a prayer to Mary.

Not necessarily. It might well be that God intends to honor Our Lady by granting the favor I seek through her intercession in a particular way. In that case the grace is to be given through her provided I honor her by addressing myself to her. Again, every prayer to Mary is in reality the asking of a favor from God even as the mother of Christ is requested to ask the same favor also. It is often better to ask God for a favor and to have someone else praying to God with one for the same favor. Two prayers are better than one. And above all, when the other one praying is Christ’s own mother.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty