[mlyle]

Heritability of intelligence is high, and there's a few genes already existing in the human population that are well correlated with intelligence.

You could just try turning a few known knobs and see what happens. The efficacy is probably moderate-- maybe 7 IQ points or so with the currently known genes. But a change of mean IQ by 7 is actually a pretty substantial advantage-- it's half a standard deviation, and predicts about $4k more income per year.

Yes, it's unethical and there's probably substantial human cost.

Or, there's even a shortcut of just testing for people with high IQs that also possess the best known genotypes (i.e. test both genotype and phenotype) and forcing them to breed.