I don't see why they can't raise the "optimal balance", you can raise brain performance, but what's to say they can't easily control other elements of the AI-human brain, sort of a self-aware safety net within the brain itself(yes I think I've seen this in SF before)that eliminates by products of increased peformance like schizophrenia. Or who is to say a virtual human/foglet brain simply isn't much hardier than a totally natural or augmented biological brain?
Are you arguing from science or from the desire to believe? Too many people cling to the Singularity as a matter of religious faith -- "the Rapture for geeks," as Ken McLeod calls it. Science demands healthy skepticism. And just in general, the future never turns out to be the way people expect it to be. With so many people today utterly convinced that the Singularity is inevitable, the more that convinces me that it won't happen, certainly not the way people expect.