For me, it was fairly substantial. Eco for some 70% highway journeys I was getting a consistent 32-33.

I tried out Comfort instead and hit 38mpg a couple of times now on long (50+ mile) trips. Certainly you have to choose to drive it carefully (as opposed to Eco where you are pretty much forced to drive sensibly) but I wasn’t manually coasting or anything special, just being smooth and gentle.

Maybe this is a unique 540ix observation.

Totally agree with this. My g30 540ix is more economic on long drives in Comfort than in Eco Pro when driven carefully.

I find it’s limited in effectiveness. When I first bought this car (540ix) I did a few 80 miles round trips, 70% motorways, and each trip returned me 32-33mpg. I stopped using it as an experiment on a few more similar trips and managed to see 38mpg pretty consistently. The only time I’ve found it to be better is in town driving. Otherwise it’s Comfort and a (mostly) light foot all the way.