> How far from optimum the human mind is, and how complex it is to improve upon it.

Nature is pretty good at optimization. We can often build devices that out perform animals on certain characteristics, but that's because we've limited the scope of what they do and we aren't concerned with the energy requirements. We also know that, as interesting as our brains are and as powerful as they are at certain kinds of problems, they aren't the best at all of them. For instance, chimps appear to have a significantly better short term working memory. It is possible that we are already close to optimum for the kind of intelligence we are interested in. I suspect that if we manage to significantly improve upon it, it will be because we don't have to concern ourselves with the energy requirements.