Now now, boys and girls, lets not fight who has the biggest d alright?
It is true that we only use 10% of our brain to function "normally" as we know it.
It is also true that the rest of the 90% could be used.
People can use more than 10%, but they would lose some normal functionality with this. You can see this in people who are partially disabled like Autism, in some cases they use more percentage of one side of the brain more than usual, but it also deflects on their other side.
In overall our brain is made in such way that we could be Mad Scientists, but we would sacrifice other things which are normal to us now.
Also, I guarantee in 100+ years if the technology progresses, then our brains will become supercomputers.