This
idea is easy to challenge on
evolutionary grounds: Why would we evolve an organ that is only used to 10% capacity?
As any decent text on human evolution will tell you, our heads and brains are disproportionatly large as compared to other animals. This is not without costs and design trade-offs. See the significant costs of the large human brain.
Due to these costs, we should conclude that the large brain must have had strong benefits in order to evolve to that size. Any unused capacity would have been eliminated a long time ago.
Quite simply, if we could have had smaller heads or brains, even slightly, even 1% never mind 90%, without sacrificing intelligence, we would have.