My rough translation. The Dominican Fathers translation is here; a version of the Latin can be found here. This article should be contrasted with Leibniz's arguments that God could not have made better.
We proceed in this way to the sixth. It seems that God is not able to make better that which he makes.
 For whatever God makes, He makes in a supremely powerful and wise way (potentissime et sapientissime). But something is much better to the extent it is more powerfully and wisely done.
 Further, Augustine (in Contra Maximin.) argues in this way: if God were able, but willed not, to beget a Son equal to Himself, He would be envious. For the same reason, if God were able to make something better than He has made, but willed not to do it, He would be envious. But envy is in every way removed from God. Therefore God makes everything the best. Therefore God is not able to make something better than He makes.
 Further, what is best and very good, is not able to be made better: because nothing is better than the best. But, as Augustine says (in Enchirid.), each thing is good that God has made, but the whole universe very good: because the admirable beauty of the universe consists in them all. Therefore the good of the universe is not able to be made better by God.
 Further, the man Christ is full of grace and truth, and has the Spirit immeasurably (non ad mensuram): and so He is not able to be better. And created happiness is said to be the highest good: and so it is not able to be better. And the blessed virgin Mary is exalted over all the choirs of angles: and so she is not able to be better. Therefore not all the things that God has made can be made better.
But on the contrary is what is said at Ephesians 3:20, that God is able to do all things more abundantly than we desire or understand.
I respond that it must be said that the goodness of anything is double.
[A] One which is of the essence of it, as being rational is of the essence of man; and with regard to this good, God is not able to make a thing better than it is itself, although he can make something else better than it. Just as he is not able to make the number four greater: because, if it were greater, it would not be the number four, but some other number. And in this way one has the addition of substantial differences in definitions, as the addition of units in numbers, as is said in Metaphys. VIII.
[B] Another goodness is that which is outside the essence of the thing; as the good of man is to be virtuous and wise. And according to such good, God is able to make things better.
But simply speaking, God is able to make something better than anything He has made.
To the first, therefore, it must be said that, when it is said that God is able to make something better than He makes, if 'better' is nominal, it is true: for He is able to make something better than any thing. Likewise He is able to make the same thing better in some way, and in some way not, as is said.If 'better' is adverbial, where it imports the manner of making (modum ex parte facientis), God in this way is not able to make something better than He makes: because He is not able to make it out of a greater wisdom and goodness. But if it imports the mode of the thing (modum ex parte facti), in this way He is able to make it better: because He is able to give the things madesome better mode of being as regards their incidentals (accidentalia), although not as regards their essentials.
To the second it must be said that it is of the notion of a son to be equal to the father, when he is matured (ad perfectum venerit): but it is not of the notion of any creature, that it be better than God has made it. Wherefore there is no analogy.
To the third it must be said that the universe, supposed as these things, is not able to be better; on account of the beautiful order given to things by God, in which the good of the universe consists. For if anything were better, the proportion of the order would be corrupted: as, if a string were more tight than was intended, the melody of the lyre would be corrupted. But God could make other things, or add other things to these things made: and in this way that universe would be better.
To the fourth it must be said that the humanity of Christ, given that it is united to God, and created happiness, given that it is enjoyment of God, and the Blessed Virgin, given that she is Mother of God, have in a way an infinite dignity, from the infinite good that is God. And in this sense there cannot be something better than these, as there cannot be something better than God.