As you can see from my numbers, P&G pays off well in this case. The last trip was on highway at about 90 km/h average speed. I like to find myself a big semi behind which I can pulse & glide into the draft. This gives you some idea about the average speed. The highest gear (5) results in about 2000 rpm at that speed.
Interesting about the intercooler...
The chip trimmer company mentioned above actually achieves the results partly by replacing the intercooler with a larger one.
See:
Power Kit L200 Triton 2,5 DI-D
They refer to less pressure drop so maybe the same effect is had by removing it altogether.
The question remains...
What happens with efficiency during the lag phase?
There is no black smoke or anything so perhaps the ECU does the best of it.