I've seen chiptunes tested on rolling roads, and most of them came out with better performance (meaning a smoother curve), and afterwards most of them had better mileage. And I'm talking about turbodiesels, these aren't modded in any other way except chiptuning, so I doubt it's the sound making the difference.
My dad thinks the same way like some here do: if a manufacturer could get higher economy and better performance, they would have done it already. But if you get a good chiptune, it'll get tuned for that specific car, every engine is different and standerd chips need to cope with every engine they build and cover the differences without too much notice.
I can't see how so many people (also people really into cars) can be so positive about chiptunes, and then I see other people who just have an idea about it, looked at people who did it wrong and go spread the word chiptuning is evil.