John Von Neumann probably isn’t the ceiling, but even if there was a near-human ceiling, I don’t think it would change the situation as much as you would think. Instead of “an AGI as brilliant as JvN”, it would be “an AGI as brilliant as JvN per X FLOPs”, for some X. Then you look at the details of how many FLOPs are lying around on the planet, and how hard it is to produce more of the, and depending on X the JvN-AGIs probably aren’t as strong as a full-fledged superintelligence would be, but they do probably manage to take over the world in the end.
John Von Neumann probably isn’t the ceiling, but even if there was a near-human ceiling, I don’t think it would change the situation as much as you would think. Instead of “an AGI as brilliant as JvN”, it would be “an AGI as brilliant as JvN per X FLOPs”, for some X. Then you look at the details of how many FLOPs are lying around on the planet, and how hard it is to produce more of the, and depending on X the JvN-AGIs probably aren’t as strong as a full-fledged superintelligence would be, but they do probably manage to take over the world in the end.