Actually, the ECUs these days are "smart" enough to adapt to all those factors you mentioned. In the case of the Miata, it was well-known that there was more performance to be had. It cost some fuel economy, but not as much as you might expect. It accelerated like a motorcycle. The only problem was, after teasing out the extra pep, the first gear was much too low. Some guys went so far as to change out the rear end for a taller ratio but I never got that far...ECU's are typically mapped to provide the best driving experience and mileage across all driving conditions which is why I mentioned that sure, it could help, slightly. In cases like yours with a Turbo they are often "detuned" for a variety reasons, meet cafe/fuel standards, to not beat out more expensive models from the same manufacturer, safety, or whatever. Typically though most manufacturers will default to fuel economy over power when mapping an ecu, exactly what you noticed. You got more power but lost economy. This is why the idea of 35% is simply ludicrous. 1-2% sure that is possible, the mapping on these things has to take into account multiple octane ratings, altitudes and weather conditions, etc. In theory if the ECU was customized to a locality and individual there may be some fuel gains.