It's a metaphor for general ability, not a concrete estimate of actual physical brain usage. People who use it tend to mean "You, yes <i>you</i>, could amazing and magical things if you were more focussed."<p>That's very unlikely to be literally true in the way they hope. But given how poorly most people are educated, and how often they're fed lies and distortions in adulthood, there's likely to be some metaphorical truth to it.<p>So the population as a whole certainly could be smarter, more aware, more effective, and probably more creative too.<p>But it's not something you could easily put a number on. Certainly a one dimensional measurement like IQ won't capture it.