you are actually asking 2 questions here. a turbo forces air into the engine and allows it to burn more fuel and make more power. by itself that is no more efficient than any other engine consuming the same amount of fuel.
engines with less cylinders are more efficient because they have fewer moving parts causing friction losses and smaller cylinders burn their fuel more completely. 4 cylinders in cars are always inline, but you could make a v6 engine with smaller cylinders. this would not help because going v configuration adds complexity and more parts, so less efficient.
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