To begin with, there are significant risks of medical complications—including infections, electrode displacement, hemorrhage, and cognitive decline—when implanting electrodes in the brain.

This is all going to change over time. (I don't know how quickly, but there is already work on trans-cranial methods that is showing promise.) If we can't get the bandwidth quickly enough, we can control infections, electrodes will get smaller and more adaptive.

enhancement is likely to be far more difficult than therapy.

Admittedly, therapy will come first. That also means that therapy will drive development of techniques that will also be helpful for enhancement. The boundary between the two is blurry, and therapies that shade into enhancement will definitely be developed before pure enhancement, and be easier to sell to end users. For example, for some people, treatment of ADHD spectrum disorders will definitely be therapeutic, while for others it be seen as attractive enhancements.

Not only can the human retina transmit data at an impressive rate of nearly 10 million bits per second, but it comes pre-packaged with a massive amount of dedicated wetware, the visual cortex, that is highly adapted to extracting meaning from this information torrent and to interfacing with other brain areas for further processing.70 Even if there were an easy way of pumping more information into our brains, the extra data inflow would do little to increase the rate at which we think and learn unless all the neural machinery necessary for making sense of the data were similarly upgraded.

The visual pathway is impressive, but it's very limited in the kinds of information it transmits. It's a poor way of encoding bulk text, for instance. Even questions and answers can be sent far more densely with a much narrower channel. A tool like Google Now that tries to anticipate areas of interest and pre-fetch data before questions arise to consciousness could provide a valuable backchannel, and it wouldn't need near the bandwidth, so ought to be doable with non-invasive trans-cranial techniques.