Cassius falls under the category of weakly powered caster whose long life span gave him enough time to develop a talent with some teeth. Other than the death curse the only spell I remember him casting without the coin was creating the snake construct that Mouse fought. He was able to do that with no focus items or preparation very quickly, which speaks to at least a moderate talent.
With Salurial, he did plenty. In fact, didnt Harry suspect that he was largely to blame for actually casting the entropy/Barrabus curse that forces him in doors with Susan?
Point being, he had plenty of knowledge on how magic worked (from a very ancient teacher) but very little power to actually get things done. I think we are all more concerned with his death curse than Jim cares about writing it in.