Sugar, like all carbs, is less efficient per weight (carbs = 4 cal/gram, protein 4 cal/gram, and fats 9 cal/gram). But if the moisture and fiber content is low (as I assume it is) then it is a rather efficient carb. The only problem is, like you said, that it is a simple sugar, and thus will give you a spike of energy and then fade quickly. I think that is why it can upset your stomach if you eat too much. But that doesn't mean it isn't efficient. So far as I can tell, table sugar is pretty much all carb (no fiber, no moisture) in that it is close to 4 cal/gram:
By the way, this chart (which I've seen repeated many times on the web) doesn't add up:
Table Sugar, 1 level teaspoon (4g) 15
Table Sugar, 1 heaped teaspoon (6g) 25
15/4 = 3.75 while 25/6 = 4.166 (obviously there is some rounding going on here). Anyway, my point is -- if you just looked at the numbers (these numbers) you would say that table sugar is a very efficient way to get your carb calories. I think we both know that wouldn't be a good idea, even if it is efficient.
Oh, and these: http://www.kendal.mintcake.co.uk/ are almost all sugar and they were on the first successful Everest expedition. Despite that success record, I wouldn't recommend too many of those.