I've been studying this question recently, occasioned at first by a conversation with my wife, right before it providentially popped up in my daily philosophical readings--in Hugon's Cursus Philosophiae Thomisticae, Vol. 3 on Metaphysics.
The Thomistic response may sound impious to some, but it is affirmative. The alternatives are either that Our Lady is infinitely perfect, or that God is not omnipotent, neither of which will stand.
She is perfect as a creature, and the greatest creature God actually created, "higher than the cherubim, and more glorious beyond compare than the seraphim" (as the Byzantine hymn Axion Estin proclaims), yet not the greatest creature possible for God. There is no such thing as the greatest creature possible for God--it is a contradiction in terms.
Here is an excerpt from Hugon's Cursus Philosophiae Thomisticae, which I'm currently translating into English, Vol. 3: Metaphysica, p. 430. (The original Latin work is available from ITOPL):
III. – Second Conclusion: “It is impossible for a creature to be most perfect of all [possible creatures].” This is the view of St. Thomas, Suárez, and the Scholastics in general.
A creature, no matter how perfect, is infinitely distant from the participability of the divine perfection, which can never be exhausted. But between infinitely distant things there can be an infinite number of intermediaries. Therefore, between God and the most perfect creature there can be an infinite number of intermediary creatures that participate more and more in the divine perfection; and there will never be a creature that fully exhausts the divine participability.
– You will say: God knows the most perfect of all [possible] creatures. But God can produce what he knows. Therefore.
– I respond: I distinguish the major: That God knows that creature as something outside of the series of possible things, I concede; as something within the series of possible things, I deny. I contradistinguish the minor, and I deny the consequence. – Such a creature is impossible. Therefore, it is known by God as something outside the series of possible things.