On models without Drive Mode Selection (i,e up to and including SE) there is no option to select ECO mode. However on ACC, you can chose "ECO, COMFORT, NORMAL, SPORT". I thought this related to the throttle response only. i.e when adaptive cruise accelerated it picks up very slowly in ECO, and quite rapid in SPORT. However, I have tried it in ECO, and came up to slow/standing traffic. I did notice that rather than slowing down gently as usual with little braking, the car came up behind the slow/stopped car quick quickly, and then put the brakes on quite hard (It was quite disconcerting, as I felt I needed to jump on the brakes myself!, and you dont usually get this feeling)
Now you mention engine braking it seems quite likely this is the difference. In theory, not using ECO should reduce brake pad usage . It seems likely that ECO model on ACC, is actually running the same logic, as Drive Mode Selection would (i.e slow throttle, less engine braking )