If the ground is flat I would expect little if any upside. Your mass is losing speed while coasting or call it momentum energy. You have to recover it at some point.
In theory if your overall trip speed is reduced there might be a small gain. As one poster mentioned if the injection pump leans out a little at 1000 rpms by design is another consideration. Yes I agree these engines use little fuel at idle when at operating temperatures.
Best mpg gains are probably gotten by keeping the engine tuned up properly from time to time and using synthetic oil. Especially if most trips are very short and local in nature.
Does that thick dino oil ever drag these 240ds down until quite warm in my experience. Never driven another car with as pronounced an effect. That is a lot of fuel milage lost right there.