Slade Wilson has as one of his superpowers “the ability to use more of his brainpower than the average person.”   This is kind of a useless power though.   It is a common misconception among people to think that humans use only 40% of our brain power.    This is true but only if you understand the context of the statement.   The human brain has evolved over our history to be quite an efficient organ.   In fact we have sufficiently smaller brains than cro-magnons but ours are more efficient making us more intelligent.   What we wouldn’t have evolved to do is to have an organ which is 60% useless.   Imagine that your heart can pump 150% percent more blood, but just doesn’t.   The extra muscle that our body would have to produce would be wasted for an organ that wasn’t justifying so much input.   This holds true with the brain as well.   So when they say that humans use only 40% of our brainpower its not really accurate.   The average human in the span of a regular day will use about 99.9% of their brainpower, just if you used more than 40% at a time your brain would get fried and you would probably be clueless as to what to do.   Perhaps some people are better at using their brain power in different ways – in fact some say that the differing viewpoints that people can bring to a conversation are quite valuable if they use different parts of their minds.   For instance there are some novel solutions to problems if you are using the creative part of your mind and not just the analytical.   Still even then you wouldn’t be using anything close to 100% of brain power because you couldn’t .   Brain power includes every single part of our brain from the most refined parts down to the more basic amygdala.   So to describe this another way, imagine playing a complicated piano arrangment while screaming at the the music because you find it terrifying.  Its not much of a power.      

I had to look it up to verify what was truth versus myth.  This is what I found in Wikipedia... 

"People do not use only ten percent of their brains. While it is true that a small minority of neurons in the brain are actively firing at any one time, the inactive neurons are important too. This myth has been commonplace in American culture at least as far back as the start of the 20th century, and was attributed to William James, who apparently used the expression metaphorically. Some findings of brain science (such as the high ratio of glial cells to neurons) have been mistakenly read as providing support for the myth."
 
This is a power that could probably be re-explained in a manner that is not so easily refuted by fact.

" @RazzaTazz: yea deathstroke is stupid.  Deadpool is way cooler.   "

Couldn’t they just say he is smart to make it easier? Besides when you put more usage of your brain it would depicts that he has psychic powers.

as someone who has taken a neuroscience class I can safely say people are using all of their brains. The whole humans only use 10% of their brains theory is just wrong, as the OP alluded to

I'm pretty sure that now it's been stated or just generally accepted that his brain processes information nine times faster than normal.

@RazzaTazz: exactly. People get their energy from sugar (glycolysis and other metabolic pathways make and use ATP ). Than our veins carry/circulate the oxygen and dissolved sugars in our blood, feeding/fueling/powering our brain. Generally speaking the mental processes take so much energy the brain needs to be well fed and rested in order to function optimally.

Mechanically we use 100% of our brains. I've heard the other argument that we use our brains but don't maximize its use, like a stereo set to a low volume decible. Even than the 10% theory is still ridiculous. There is no tangible data I know of to determine the amount of usage the human brain has. We know of rare cases of people showcasing unique mental functions, such as photographic memory or individuals with autism that can ability to calculate high level mathematical equations without learning how to. We use separate parts of our brain for different functions, I'm sure you've heard of left and right brain theory.

Our brains are our most amazing ability and the average individual can probably optimize their usage to an extent. But as you and others have said or alluded to the notion that we simply don't use a large percentage of our brain is a misnomer