Well - - I understand the logic, but some (4) years ago, as a member of the MBenz Club, I had my 2000 S500 ECU remapped to give a better fuel economy. I'd run the car for a couple of years, and achieved an average annual 30.2 mpg (I'm a steady driver)
The 'engineer' running the laptop explained that as these engines are mass-produced to cope with different petrol standards across the globe, they leave the factory on 'one-size-fits-all' settings on the ECU.
What he claimed he would do would be to optimise the ECU for UK 95RON fuel.
Well - over the following three years, the car returned an average 34.1 mpg, and was certainly even sharper on acceleration. (I cannot comment on upper its speed ranges as the blasted thing would reach 155, and I'm not that stupid on the roads!)
So I regarded that as a success.