Hmmmm
Iv never heard of having an ECU reset before before being plonked straight back into a car with significantly higher mileage than a new ECU would initially understand

Still bearing in mind the ECU is a small computer, they must be prone to getting slower with age as per your off the shelf desktop, as it stores all the little anomalies that occur throughout its time.
If the performance is anything other than psychological, it is where it has yet to take account of the aged sensors that are giving it its readings.
Unfortunately were technology has advanced over the years, and the budget differences between the engineering/design teams at M-B compared to Ford, I doubt any such advantages would be so prunounces in the Fiesta

Might be wrong though.....
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From the moment I picked up your book until I laid it down, I was convulsed with laughter. Some day I intend reading it.