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Post 60 is a BSFC map of a N/A Saturn DOHC motor, and your hypothesis of what would happen if it were turbocharged. For a fuel economy increase to occur, that island needs to be moved down, not left, right or up. Turbocharging stretches it upwards and only upwards. This is because until you start hitting boost, the turbo does absolutely NOTHING for your engine. Do you understand what torque is? My car does not need 130lbft of it to maintain a 55mph cruise on flat ground, therefore, I don't spend much time in that island unless i'm climbing a mountain. If I do operate in that high efficiency region, I'll soon be going so fast that any efficiency gains at the crank will be eaten by wind resistance.
We're talking apples to apples here, slapping a turbo on THE SAME motor. Doing so will not improve fuel economy, and will certainly not reduce cost per mile operation as premium fuel is often required.
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