Human intelligence is primarily limited by the 6-10 item limit in short-term memory. If you bumped that up by a factor of 5 we could very easily solve vastly more complex problems, and fully visualize solutions an order of magnitude more subtle and messy than humans can manage today.

That's a relatively easy thing to do architecturally once you have a model that can match human intelligence at all. TBH if we could rearchitect the brain in code we could probably easily figure out how to do it in ourselves within a few years, but our wetware does not support patches or bugfixes.

We can't improve ourselves, but that's only because we're meat, not code. And of course no AI has done it yet, because we haven't actually made intelligent AI yet. The question is what happens when we do, not whether the weak-ass statistical crap that we call AI today is capable of self-improvement. Nuclear reactions under the self-sustaining threshold are not dangerous at all, but that was not a good reason to think that no nuclear reaction could ever go exponential and be devastating.