Thanks for a good comment. My oversimplified thought process was that a 10x increase in energy usage for the brain would equate to a ~2x increase in total energy usage. Since we’re able to maintain that kind of energy use during exercise, and elite athletes can maintain that for many hours/day, it seems reasonable that the heart and other organs could maintain this kind of output.
However, the issue you bring up, of actually getting that much blood to the brain, evacuating waste products, doing the necessary metabolism there, and dealing with so much heat localized in the small area of the brain, are all valid. While it seems like the rest of the body wouldn’t be constrained by this level of energy use, a 10x power output in the brain probably might be a problem.
It’s worth a more detailed analysis of exactly where the max. power output constraint on the brain, without any major changes, lie.