>We can look at efforts to stimulate the brain, which do not seem to scale easy.
Simulating the brain isn't necessarily the path to human-level AGI though. Simulating biology in digital form tends to be inefficient.
Modern machine learning scales easily, or at least relatively easy.
>(and it could theoretically be detrimental; look at what happens to humans who have too many neuronal connections)
Perhaps a constraint of our brain's cognitive architecture and not intrinsic to cognition itself. Still, interesting point.
And that's just talking about a raw increase in computing power. We don't have any idea whether a human level intelligence program would even be able to put increased processing power to good use (and it could theoretically be detrimental; look at what happens to humans who have too many neuronal connections).