It's +0.1 mpg. And it's just for the idle speed.
Thank the last poster of comments on that page for the source.
"The noise-cancellation system also allowed engineers to drop the idle speed of the I-4, which yielded about 0.1-mpg-better fuel economy."
It's could be a warranty issue, people who hear the noise might assume something is wrong and take the vehicle in claiming the engine is damaged. But even that's far fetched.