I also think we will reach a limit when it comes to intelligence augmentation. Artificial intelligence will never have a limit and it doesn't have all the evolutionary baggage we have.
An AI can be a rational agent. It doesn't have to fight impulses, temptations, attention control, exercise emotional regulation etc. It is not stuck in a body limiting it and putting constraints on its time.
For now, research on AI and IA go somewhat hand in hand. We still don't really understand what differentiates us from intelligent animals other than the ability to handle higher complexity.
AI researchers focused on replicating every brain module in the hope it will become intelligent are most likely to create a smart animal but nothing comparable to human. Looking at our ancestors, they were able to create tools, fire, communicate etc. Hell, neanderthals could copulate with us.
Something happened in our brains between the age of neanderthals and us. 99.5% similarity and if we could find what that 0.5% is, maybe we could focus on enhancing/replicating that instead of every brain module. People speculate it is creativity (divergent thinking) since art appeared in caves but there was none prior to that. The language gene was present in neanderthals and so what the ability to create tools, cooperate in groups to hunt etc.
The fear of AI destroying everything is a genuine one. If we create something as smart as a bear, it still wouldn't be smart enough to compete against us in every arena but like a bear, it can use its sheer power & speed to overwhelm us.
PS: I find the subject of neanderthals fascinating, if anyone has a good recommendation on the evolution of intelligence or finding what that 0.5% is, please let me know.