ECO really just softens the input. I would gather the mpg gains to be minimal as the real differences are whether you're mashing it all the time and braking really hard. Traffic gets me in ECO as it gives me a little bit of space before it kicks the engine on. Same in decreasing speed ... it cuts the engine off basically under 40 mph depending on the active state of the vehicle (just coasting to a stop). Having it in norm and subsequently sport to S+ changes the limiters to where those items click, not necessarily the actual mpg.

 

In my car, ECO mode works great within the city. It would up-shift earlier and throttle is significantly reduced leading to better gas mileage. On the same route, i can see a 2 - 3 mpg difference. Not significant but it does highly depend on driving styles. I'm seeing about 21 mpg in the city in 20 - 30 degree climate in a premium AWD.