The logic that we are using "only a small part" of our brain is caused by the sparse activations. When perceiving something, we need to encode the raw sense data. When encoded, only a small part of the brain is necessary to represent it, thus the misconception that we are only "using" that part. In fact we are using all of it, even the inactive parts. By analogy, when I am writing, I am only using one letter at a time. This does not imply that I am only using 4% of the power of the alphabet, just that it works by sparse combinations. Thus there is no basis to thinking that we could improve if only we could use 100% of our neurons at once.