Anyway, let's assume for the sake of the argument that science really didn't proceed slower in that possible world. Now the next question is: who does all the discoveries that von Neumann did in the actual world in this possible world? If it's a guy as smart as von Neumann, then it seems your argument doesn't defeat my thesis (that beyond a certain level of ability, productivity increases massively). If it's a guy who's not as smart as von Neumann then that would be a problem for my thesis - but it seems to me unlikely that that would have happened.

What if it's several guys rather than "a guy"? Then it is possible that each one of them is not as smart as von Neumann and each one makes fewer discoveries than him, but all together they make all of his discoveries.