The thesis under examination as expressed by Diogenes Allen: "The world plus God is not more than God alone. God less the world is not less than God alone." Is this a defensible position? Let's consider both sides of the question.
A. First, a crisp little argument against the view.
Consider two possible scenarios. In the first, God alone exists. In the second, God exists and creates a world. On a classical view of God, according to which he is libertarianly free, both scenarios are indeed possible. There is no necessity that God create; his creating is free in the 'could have done otherwise' sense. Clearly, the scenarios are different. But if God + World = God, then there is no difference between the two scenarios. For on that supposition, God alone exists in both scenarios. Therefore,it is not the case that God + World = God.
To extend the argument:
If God is Being itself, ipsum esse subsistens, Being in its plenitude and infinity, then how could there be anything else? If God is Being itself, and thus not a being among beings, how could there be any 'ontological room' for anything else? How is creation so much as possible if God is Being itself? Isn't the Thomist line, as articulated by Diogenes Allen and Etienne Gilson (quoted previously) just obviously mistaken?
After all, it is evident to the senses (though not self-evident, cf. Descartes' Dream Argument) that this material world of time and change exists: it is not nothing. Nor it is a dream or an illusion. Clearly, it is 'better known' that this material world of multiplicity exists than that God exists. But suppose God does exist. Then both the world (creatures) and God exist. Is it not perfectly obvious that the totality of reality is greater with both God and creation than with God alone?
B. Now let's consider what could be said in favor of the view.
Given the force of the arguments for the thesis that God is not a being among beings, arguments we cannot rehearse again here, it is reasonable to hold that God is Being itself. This leaves us with the task of attaching some tolerably clear meaning to 'God + world = God' in the teeth of the argument contra. This cannot be done if there are no modes of Being. For if everything that exists exists in the same way (mode), and if G exists and W exists, and they are numerically distinct, then it is self-evident that there is a totality of existents and that this totality is greater if G and W both exist than if G alone exists.
So we need to bring in modes of Being or existence. To motivate the modes-of-Being doctrine, consider an analogy. I am standing before a mirror looking at my image. How many men? One, not two. I'm a man; my mirror image is not a man. An image, reflection, picture, drawing, sculpture of a man is not a man. And yet my mirror image is not nothing: it exists. I exist and my image exists. Both exist, but in different ways. I exist whether or not any mirror image of me exists; but no mirror image of me exists unless I exist. Note too that the mirror image is dependent on me for its existence at each moment of its existence, unlike a photograph or a sculpture. (Herein an analogy with creatio continuans.)
It is also worth noting that there is a correspondence between the visual properties of the man and the visual properties displayed in the image. (This fact is what allows a dentist to do precision work on a tooth without looking at it directly.) Now we cannot say that the seen man and his image instantiate the same quidditative properties since, e.g., the man is bearded but his image is not. But we can say that the same visual properties instantiated by the man are displayed in the image. While the image is not bearded, it is an image of a bearded man. There are two different properties, but they are related: being bearded, being of something bearded, where the 'of' is an an objective genitive.
Man and image both exist. Yet there is an important difference. I say it is a difference in mode of existence. The image, unlike the man, exists dependently or derivatively, and it depends existentially on the very original of which it is the image. Existential dependence is not a quidditative property. This mode of existence is no more a quidditative property than existence is.
So I say we need a tripartite distinction: quiddity (nature, essence in the broad sense); general or quantificational existence, the existence expressed by the particular quantifier; mode of existence.
Now it makes a certain amount of sense to say that Man + Mirror Image = Man. This could be explained by saying that there is no totality of independent existents that has both me and my mirror image in it. If we are adding and subtracting over a domain of independent existents, then it is true that Man + Image = Man.
Accordingly, 'God + World = God' could be explained by saying that there is no totality of a se existents that has both God and creatures in it.
C. Aporetic Conclusion
The argument I gave in section A will strike many as compelling. But what I said in section B shows that it is not compelling. If one holds that God exists in a different way than creatures, then there is no totality in reality to which God and creatures all belong. One can of course say that something is (identically) God and that something is (identically) Socrates and that *Something is (identically) ____* has exactly the same sense, no matter what you throw into the gap: no matter what its mode of Being. But that implies only that there is a merely conceptual totality to which God and creatures all belong. In this merely excogitated conceptual totality, however, abstraction is made from the real existence of the things in question, and their different modes of Being.
I grant that God and Socrates both exist in the quantificational sense of 'exists,' a sense univocal across all existential sentences regardless of subject matter; but that is consistent with there being no commonality in reality between God and creatures to warrant talk of a totality in reality containing both.
My interim conclusion is aporetic: both positions on our question are reasonably maintained. They cannot both be true, but they can both be reasonably upheld.
I would be satisfied if Dale Tuggy and the 'supreme (miniscule) being theists' would agree with me and other '(majuscule) Being theists' that it is a stand-off.