OK, I put it to the test. I had a big ride ahead of me and lowered the speed by ~3 mph - 59 MPH (95 km/h) instead of my usual 62 … From time to time I kept driving behind trucks. It was quite hilly because it mainly went through the Ardennes - known in the US for the toughest battle in the second world war. So I called the trip my Battle of the Bulge -
a tribute. 
Result: 78.4 MPG = 3.0l/100km, turning my car into a Lupo/A2 3L car
Distance: 144 miles
Average speed: 50 MPH
Direction: back and forth
Weather: dry, 22°C, 10 MPH wind NNE
90% highway at 59 MPH, 10% country roads
That's an instant improvement of 4,5 MPG. I think this is about the lowest consumption possible with my driving behavior. I've got some more mods to do first so we'll see.