Well, if by 'use 10% of the brain', you mean only 10% of neurons can fire at once, then increasing usage will cause an increase in energy consumption and tempurature.
On the phsychological side, I think problem solving and multitasking will increase up to a point. above 50% it will probably decrease again as the brain becomes congested (there are not enough routes through the neurons, limiting intelligence)
If you mean how many of our neourological proccess we can consciously control, then an increase in usage will result in exponentially increasing intelligence, as we conciously find solutions for the shortcomings of the brain (i.e, if you can control your neurons, you can have an almost infinite number of parrelel thought proccesses).
Psy-powers? Not impossible. You may be able to read and control minds, if the brain can pick up and send electromagnetic fields from and to other brains. This would also allow mental communication - your brain's near-infinite parralel thought proccesses allow you to decode anything in a matter of seconds (or less), enabling you to map neorons firing into other peoples thoughts.
Controlling physiological procces would also become possible (to a limited extent), as you would be able to control the primitive areas of your brain that are responsibe for reflexes and homeostasis. This would mean you could control anything to do with homeostasis (heart rate, blood pressure, tempurature, metabolism, growth and healing <-- within biogical boundaries)
as well as reflexes (reactions to pain and fear, pupil dilation, heart rate, blood pressure, adrenalin production, etc)
You would develop a photographic memory, as you made sure each memory was carefully stored, and not altered by anything else.
I do not think changing DNA, omnipotence, tele/pyro/kenesis or altering reality would be possible - but the brain is an incredible and mysterious organ, so who knows?