Steel_Wind, I don't think the evidence that you described necessarily means what you think it does.
Sure, the brains of people who suffered that syndrome may have less brain mass and volume. However, the brain's mass and the brain's capability are not equivalent. It seems possible to me that a brain that has been reduced to 1/10th of its mass may very well have just as many living neurons and neuron connections as a healthy brain. After all, large amount of brain tissue is just insulating fat, fluid, and other such things that are useful, but not strictly necessary. As such, I can believe that the overall structure of the brain can be compressed without actually damaging its operative capability.
In other words, brain size and brain mass are not directly proportional to the number of neurons, neuron connections, and intelligence. A person with a large head (and thus a large brain) is not necessarily smarter than a person with a small head. In fact, arguing that such a thing is true is laughable. Also, it is not like everyone is born with a brain that is at its adult size, yet the growth of a person's mental faculties as they grow older is not associated with the growing mass of the brain (in fact, an important part of mental development is the mass death of large numbers of neuron connections in the brain, which happens twice in every person's lifetime).
A person who has a brain that is at 10% normal size, but is still functional, is pretty much just a person who has a brain that is miraculously healthy and normal despite unusual circumstances. It is not a case of someone being reduced to 10% mental capacity, and miraculously making up for that difference using "untapped potential".
Besides, the direct correlation between "brain's capability" and "intelligence" that is inherent to the normal use of the "we only use 10% of our brains" idea is still pointless, even if I were to agree with your argument. After all, since something like 30% or more of our brains are dedicated to processing visual information and nothing else, wouldn't increasing the "brain percentage" just improve our sensory capability, muscle control, memory, and emotional sensation, rather than just intelligence?
