The quote from Egan would seem to imply that for (literate) humans, too, working memory differences are insignificant: anyone can just use pen and paper to increase their effective working memory.

I believe that what he is suggesting is that if you reached a certain plateau then intelligence hits diminishing returns. Would Marilyn vos Savant be proportionally more likely to take over the world, if she tried to, than a 115 IQ individual?

Some anecdotal evidence:

... mathematician John von Neumann, ... was incomparably intelligent, so bright that, the Nobel Prize-winning physicist Eugene Wigner would say, "only he was fully awake."

I have known a great many intelligent people in my life. I knew Planck, von Laue and Heisenberg. Paul Dirac was my brother in law; Leo Szilard and Edward Teller have been among my closest friends; and Albert Einstein was a good friend, too. But none of them had a mind as quick and acute as Jansci [John] von Neumann. I have often remarked this in the presence of those men and no one ever disputed me.

... But Einstein's understanding was deeper even than von Neumann's. His mind was both more penetrating and more original than von Neumann's. And that is a very remarkable statement. Einstein took an extraordinary pleasure in invention. Two of his greatest inventions are the Special and General Theories of Relativity; and for all of Jansci's brilliance, he never produced anything as original.

Is there evidence that a higher IQ is useful beyond a certain level? The question is not just if it is useful but if it would be worth the effort it would take to amplify your intelligence to that point given that your goal was to overpower lower IQ agent's. Would a change in personality, more data, a new pair of sensors or some weapons maybe be more useful? If so, would an expected utility maximizer pursue intelligence amplification?

(A marginal note, bigger is not necessarily better.)