When you’re talking about outliers, it’s not an even-or situation. It’s not that being diligent is more valuable than being smart. Lots of people are smart, but the ones who are exceptionally smart and exceptionally diligent—outliers on two dimensions—are usually the most successful.

It’s also worth pointing out that people who e.g. study algebra in eighth grade and calculus in high school aren’t actually outliers; they’re maybe the top 1/3 or so of the class in terms of mathematics ability.

Newton, Euler, Darwin, Einstein, Paul Dirac, Richard Feynman… Our modern world wouldn’t exist without them. Look up estimates of their IQ. Read some of their work and try to imagine having the same level of insight and producing similar volumes of it if you devoted every waking hour to the task.

Then read up on the ancient Greeks. Even after 12 years of education most modern people wouldn't be able to measure the circumference of the Earth like Eratosthenes did hundreds of years before Christ. The ancient Greeks were pretty darn smart.

So, the closest that I am aware of is the multi-decade study conducted by, the psychologist and intelligence researcher, Lewis Terman. The study was originally called, "Genetic Studies of Genius."

You can read about it here:

https://gwern.net/doc/iq/high/2018-kell.pdf

This one is somewhat tangental, but I find, "The Munich Model of Giftedness Designed to Identify and Promote Gifted Students" to be an interesting read too.

https://gwern.net/doc/iq/high/munich/2005-heller.pdf

No, not to a person. There can be some stupendously dumb billionaires, especially since inheritance is a thing. I would however expect the average intelligence however-measured of the 100 richest "self-made" (lets just say who didn't themself inherit a generational amount of wealth) individuals in the US to be higher than a 100-person random sample of the population.