I think that Aquinas is trying to accurately describe the character of Christ's sacrifice. "Perfect" is not accurate because it implies something done just right but does not describe the extent. For example, a diamond is perfect, but it is not very big. "Infinite" is too abstract: a circle is infinite, and something that never ends in time is infinite. "Superabundance" describes a different aspect of infinity and perfection altogether: it is something that, no matter what quantity you put next to it, abounds more.
Those of us who studied calculus would remember mathematical definitions that go "for every Y > 0 there exists an X such that ...". For example, a function f is called of unlimited growth if for every Y > 0 there is an X such as f(X) > Y. This is precisely what both Paul and Aquinas are saying, -- for every offense there is a merit such that atones the offense. It is then, abundant over every offense, superabundant.
"Superabundance" describes a different aspect of infinity and perfection altogether: it is something that, no matter what quantity you put next to it, abounds more."
Oh. Thank-you.