AI versus human intelligence enhancement

I do not think it plausible that Homo sapiens will continue into the indefinite future, thousands or millions of billions of years, without any mind ever coming into existence that breaks the current upper bound on intelligence. If so, there must come a time when humans first face the challenge of smarter-than-human intelligence. If we win the first round of the challenge, then humankind may call upon smarter-than-human intelligence with which to confront later rounds.
Perhaps we would rather take some other route than AI to smarter-than-human intelligence - say, augment humans instead? To pick one extreme example, suppose the one says: The prospect of AI makes me nervous. I would rather that, before any AI is developed, individual humans are scanned into computers, neuron by neuron, and then upgraded, slowly but surely, until they are super-smart; and that is the ground on which humanity should confront the challenge of superintelligence.
We are then faced with two questions: Is this scenario possible? And if so, is this scenario desirable? (It is wiser to ask the two questions in that order, for reasons of rationality: we should avoid getting emotionally attached to attractive options that are not actually options.)
Let us suppose an individual human is scanned into computer, neuron by neuron, as proposed in Moravec (1988). It necessarily follows that the computing capacity used considerably exceeds the computing power of the human brain. By hypothesis, the computer runs a detailed simulation of a biological human brain, executed in sufficient fidelity to avoid any detectable high-level effects from systematic low-level errors. Any accident of biology that affects information-processing in any way, we must faithfully simulate to sufficient precision that the overall flow of processing remains isomorphic. To simulate the messy biological computer that is a human brain, we need far more useful computing power than is embodied in the messy human brain itself.
I do not assign strong confidence to the assertion that Friendly AI is easier than human augmentation, or that it is safer. There are many conceivable pathways for augmenting a human. Perhaps there is a technique which is easier and safer than AI, which is also powerful enough to make a difference to existential risk. If so, I may switch jobs. But I did wish to point out some considerations which argue against the unquestioned assumption that human intelligence enhancement is easier, safer, and powerful enough to make a difference.