Direct research evidence is pretty scarce at the moment. Anecdotal evidence is plenty from Immortality Institute, but we all know that anecdotal evidence amounts to little. On the other hand there is evidence for negative effects on cognitive performance from metabolic syndrome and diabetes, which effects can't be reversed by taking some sugar.
It's clear cut that sugar gives a cognitive boost, but we're not biologically accustomed to a constant limbo of blood sugar. Simple sugar was very scarce in the ancestral environment and in some ways it can be viewed as a psychoactive drug.
My approach is that it's better to first minimize long-term health risks and then try to find safe solutions for short-term problems. Treating mental fatigue with sugar is a huge long term risk and as a strategy it'll become unsustainable at some point. I'd note though that there are lots of other things that may affect mental energy/performance, like thyroid hormone levels, magnesium, Omega-3, etc.
33:33:33 is a very peculiar diet and I must say I've not yet encountered it before. 33% protein means 120+ grams of it per day, which is only common among heavy athletes and bodybuilders. My own 15 percent is also high-ish as I'm trying to get some muscle via strength training and thus supplementing with some whey. I suspect that 33% carbohydrate is still too much in your configuration (and 33 fat is too low).
Note that the whole idea is about taking very small amount of sugar at the right time. It's completely unrelated to levels that cause metabolic syndrome.
33:33:33 seems to arise fairly naturally when I try to eat reasonably limited amounts (for purely cosmetic temporary reasons; and all unrelated to the entire willpower business) of the tastiest food (most of which hits diminishing returns very quickly).
Here's some data on what people eat based on spreadsheetscripting out FAO data. As you can my current protein intake is indeed highly unusual - but then so ...
There's considerable amount of evidence that willpower is severely diminished if blood glucose get down, and this effect is not limited to humans. And a small sugary drink at the right time is enough to restore it.
We're talking really small numbers. Total blood glucose of a healthy adult is about 5g and it varies within fairly limited range. Then there's maybe 45g in total body waters. Then there's about 100g of glycogen in liver, plus yet larger amount in muscles and other organs, but which doesn't seem to take part in sugar level regulation. For comparison a small can of coke contains 33g - a really small amounts at appropriate times can make a big difference.
This leads to two issues. First, is blood glucose a good explanation for willpower deficiency and therefore akrasia? I'd say there's significant amount of evidence that some effect exists, but is it really the most important factor? Humans are complicated, science knows very little about how we work, and probably half of what it "knows" is false or at best only half-true. Caution is definitely warranted.
And the second issue - if this theory was true - and by manipulating blood glucose levels you could achieve far greater willpower whenever you wanted, what would you do? It seems that exploiting it isn't that easy, and I'd love to hear if any of you tried it before.