I would think the number of people maxing out their IQ is pretty small. There's a whole lot of work, including that done by HN readers, that's pretty rote. Not too much "deep thinking" required to clone an application, follow a tic-list, ...

Similarly, the number of people who can't be lifted to "a high level" is similarly small. I assume you don't mean "those who can't function on their own", so you're discussing the mass of people who toil along in "typical jobs for typical people". Healthy, fully-formed w/o congenital defects, thankfully, is common and usually sufficient.

Edit: these tests are culture-specific. Doubt too many of the readers of HN would look functional on one for Inuit or Japanese.