Bostrom discusses this possibility in Superintelligence, both in the form of enhanced biological cognition and in brain/machine interfaces. Ultimately he argues that a super intelligent singleton is more likely to be a machine than an enhanced biological brain. He argues that increases in cognitive ability should be much faster with a machine intelligence than through biological enhancement, and that machine intelligence is more scalable (I believe that he makes the point that, while a human brain the size of a warehouse is not practical, a computer the size of a warehouse is).