> If we are using only 10% of our brains, that means a person would be fine if the other 90% of the brain got remove >> Actually you can do much better. There are people who had 50% of their brain removed and were fine.

50% is less than 90%, that's not 'much better'.

>[https://www.gwern.net/Hydrocephalus]

It's amusing you link to gwern's page where he mostly debunks those cases.