Does econ mode in a 2012 Honda civic really do anything?

I know it shifts sooner and the ac doesn't run as hard or as good.

Not owning one, I have no idea. Perhaps you could do what smart people do. Drive it for a month on eco mode and a month off and compare

It's design is to get better fuel economy.

The biggest variable is always the driver. If you leave it in economy mode but floor it every time you accelerate, the mileage will still suffer.

Assuming you drive it with fuel economy in mind, the econ-mode can help you squeeze out slightly better fuel economy numbers.