First, you need to insert the definite article, "the":
"He is of the highest authority".
Yes, the underlined preposition phrase is subjective predicative complement of "he". Often with PPs as subjective PC, the preposition is "of" and the complement of the preposition is a noun or noun phrase:
"The frame is of steel". (noun)
"it is of no importance" (NP)
"She is of a sunny disposition". (NP)
You didn't ask, but preposition phrases can also be objective predicative complements with transitive verbs where the complement of the preposition may be a noun (or NP) or an adjective (or AdjP). Usually the preposition is "as", but one or two verbs like "take" have "for" as their prep:
"I consider him as a friend". (NP)
"I regard her as very clever". (AdjP)
"I took him for dead". (adjective)