I have no problem believing 55mpg+ is achievable in ECO mode. On a group ride/trip with friends on HD's & cruisers a few years ago, I got 52-55mpg on a couple of fill-ups on my '08. Of course, highway miles at 55-60mph, level ground, no power bursts.
A few days ago, I got 32mpg on a fill-up. Sub 45F temps, climbing, winter fuel and speed changes. Of course, a lot of it is the rider. Years ago, I had a friend who would consistently get 5-10mpg more that me when we rode together. Even when we switched bikes.
I'm sure ECO mode is a good thing, but some people are just easier on equipment.