From journalist Michael Grothaus:
“And that’s exactly what I did. The day I began my two-week refined-sugar-free diet, I thought it was mostly a pointless exercise, and that I would notice little, if any, differences. How wrong I was. By the time I finished, it was nothing short of a revelation.”
“But then on Day 6, something happened. The fogginess began to disappear along with the lingering headaches. The fruits I were now eating on a daily basis began tasting sweeter. By Day 8 or 9, I felt more focused and clear-headed than I had at any time in recent memory. This translated into greater productivity—for example, I was more engaged when interviewing sources for stories. I was better able to focus on what they were saying and could rapidly respond to their answers with new queries and reformulated ideas with a speed and clarity I’ve never possessed before. While reading a book or article, I felt like I absorbed more detail and information. In short, I felt smarter.”
Here is Dr. Greger on the addictiveness of sugar:
peakmemory 
I’m hopelessly addicted to sugar, and more interested in the negative cognitive affects.
It seems to me like the sugar-rush associated with sugar is in some ways similar to caffeine, at least to a certain extent. If so, then it seems there would be positive effects as well, like increased ability to concentrate for a limited time.
Do you know of any studies showing the effects of sugar on test-taking and other intellectual activities?
The quote you gave from the Journalist is interesting, but as an anecdote I feel it isn’t very scientifically valuable, and because it is is written by a journalist (who has a reason to exaggerate), it is even more biased.