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So, I was reading a little bit about BSFC (brake specific fuel consumption, or the amount of fuel used per horsepower) and saw that the lowest BSFC occurs at peak torque (assuming the air fuel ratio remains constant). With this in mind, we could build an engine with a turbocharger that runs at peak torque while remaining at 14.7:1 by altering boost pressure.
In the performance world, they set the boost pressure as a constant for performance reasons. This requires the air fuel ratio to be modified to prevent detonation as mentioned above. However, if we were simply to lower the boost pressure to avoid detonation, we could maintain the highest possible torque the engine is capable of @ 14.7:1 air fuel ratio. This would therefore give us the lowest possible BSFC. Now, keep in mind that this would only be at WOT (no different from any other engine), so pulse and glide would need to be used. This is also where gearing becomes so important, especially for highway driving so you don't have to P&G everywhere.
This also brings up a few other things. For example, what if your engine can handle a lot more torque? Now, you have a beast of an engine in your car and the added torque now creates wheel spin because your accelerating at (near) WOT. Well, now we're back to using a smaller engine. Also, what if your engine can't handle more torque? Well, then you won't see much in the way of gains. You could therefore look into alternatives like head modifications to supress detonation, coating pistons and chambers with ceramic coatings, etc.
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