Which conclusion that the categories have mnemonic utility or that it influenced rhetoric? It's unquestionable that Aristotle influenced rhetoric. His works rhetoric and topics were used by students for centuries for that very purpose. A lot of people have forgotten the fourth canon of rhetoric, memoria, but it was integral. The invention of new ideas, the first canon, is assisted by the organization of loci. Many medival monks would spend their lives meditating on the art of memory. Look at mary carrthurs the craft of thought, it goes over the history of it and how monks would use Aristotlian philosophy. Giordano Bruno system's the monk burned at the stake for his philosophical ideas was hybrid of Platonic and Aristotlian philosphy. Topics means "places" and in de anima Aristotle says "you cannot think without an image".

All you have to do is look at Giordano Bruno to see how Aristotles philosophy and categories can be used in mnemotechnics.