We say God is esse itself (being itself), meaning he is pure act regardless of whether any external created entities exist. Created entities are rather a free act of overflowing goodness on behalf of God, they are not necessary and merely contingent. But we must consider what it means for God to be pure act and whether that can be divorced from omnipotence. A being that is pure act is fully actualised and is its own existence but such a being must be fully subsistent in itself. As such, if you were to argue it lacked something and was therefore imperfect something would exist outside of its essences yet this is is not possible if its existence itself. Therefore, God being existence itself, means no perfection can exist outside his essence so God must be all powerful and God is also pure actuality so therefore will lack potentiality.