If you leave their mind unaltered, you just have a human. They're not smart enough to really be useful. Once you start altering it, insanity becomes a likely result.
Best case scenario, you get one person's CEV. Most likely scenario, you get someone too insane to be useful. Worst case, you have an insane supergenius.
If humans weren't useful, humans wouldn't employ humans. A Hawking brain that needed no sleep would be a good start,
The reasoning of most of the people on this site and at MIRI is that to prevent an AI taking over the world and killing us all; we must first create an AI that will take over the world but act according to the wishes of humanity; a benevolent god, for want of a better term. I think this line of thinking is both unlikely to work and ultimately cruel to the FAI in question, for the reasons this article explains:
http://hplusmagazine.com/2012/01/16/my-hostility-towards-the-concept-of-friendly-ai/