Making balls ever rounder clearly has diminishing returns, as there's a limit to roundness.

Is there a similar limit to intelligence though?

Yes, absolutely. Even human intelligence stops being useful at a certain point and becomes 'enough, provided the human is motivated and tenacious'. Intention is a better metric, but it's a serious AI weak point if not actively an achilles' heel. 'Warios' come to mind.

Intelligence in humans probably stops being useful for the individual at some level, indeed. But that might be due to being in an environment that's unable to feed that intelligence and execute on the ideas in generates.

“I am, somehow, less interested in the weight and convolutions of Einstein’s brain than in the near certainty that people of equal talent have lived and died in cotton fields and sweatshops.”

― Stephen Jay Gould

Interesting. Any evidence that intelligence stops being useful? Why do you believe this is this a bounded resource.

Numerous studies have shown that the iq/income correlation breaks down at around 130 IQ points in humans.