This.You have posted this before and it is not true. The eco mode is the only mode that will decouple the engine from the transmission during coasting on the highway to increase mileage. Now I am sure it is very minimal but it is a difference.
Maybe the G80 is different and doesn't have that feature but on the G70 Eco mode should save some fuel.
Also, ECO mode is going to use more conservative camshaft timing, ignition timing, and AFR control than other modes. That said, if you compare driving on the freeway at 75mph in 8th gear on the same stretch of road, and do a run on ECO and Sport modes to compare, ECO mode WILL actually get better mileage, all other things being equal.