I’m going to be honest, I don’t know if improving Rubsico is possible. Nature has had every reason to and billions of years. It’s optimized as far as it can go and you either lose speed or specificity, I don’t think having both is possible.
That there are no more efficient variations of it in nature just tells us that the local minimum is really deep and that natural evolution, as it is, can’t produce anything better, not even with a billion years and 10^30 organisms serving as a “brute force lab”. It’s also the kind of problem an AGI system would tackle for purely ideological reasons, i.e. to prove that it is superior to nature.