Could you not theoretically do anything? You could move atoms and electricity around if you could control everything about your brain. This would allow you to move, rearrange, or convert anything into anything.
I strongly doubt it's possible to reach that stage. Its difficult to predict the extent of the effect it would have on someone since no one has come even close to such brain power (and no one will probably). I think the only change might be that you're just much more intelligent and each part of your brain would be more efficient, but you would still be probably unable of working miracles.
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There is actually a really good reason we don't do this. You don't use 100 percent of you brain because you aren't doing 100 of everything at the same time. You're not moving every body part while talking and reading and doing math and listening and eating. Each action uses a different part of the brain so you could never being doing everything so you couldn't ever use all of your brain.