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It can be hard to make a more powerful engine more fuel efficient. An ICE tends to be most efficient at higher loads at lower RPMs. So you throw in a big block and you're allways running at very low loads unless you hypermile and pulse and glide everywhere you go.
A forced induction engine will usually need to have a lower compression ratio that will also lower its fuel efficiency, not to mention when you hit the throttle it will need to enrich. Or you need to buy premium, or all of the above.
One solution is go with the smaller engine with forced induction and use water injection so you don't need to lower the compression ratio or premium fuel or have a highly enriched mix at higher loads. But be aware that if the water injection fails, bye bye engine.
You can also run and engine at a "higher load" so to speak with a well designed EGR system. Cooling the exhaust gas seems to help this even more.
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