i believe whatever capacity we think we have, we will not see. Nature' will wait til we as a species evolve and then allow the use of newly tapped brain potential as she has done historically, on top of the mental powers we have now. Lucky for the next specie of humans.
Our brains will probably need to physically form in a compatible way in accord with any new mental potential, as it has done in the past.
Believe it, or not, we do use the majority of our brains every day. We use different parts of our brains depending on what function we are performing. Just like a person does not use every muscle in their bodies at once. Each part of the brain has it's own function. It is just that some parts are used more frequently than others. It sounds false that we would not use the majority of our brains in one way or another. One way of putting it that I once read was, "A man was shot in the head, and the Doctor said, 'It's okay, the bullet only hit the 90% of the brain that wasn't used".