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Here's a downsize + turbo success story from 2002:
VW Jetta VR6
5spd Manual
Regular Fuel
200HP
2.8L
17/26 MPG
$2708 in fuel per 15K miles
VW Jetta 1.8T
5spd Manual
Premium Fuel
180HP
1.8L
21/29 MPG
$2402 in fuel per 15K miles
Turbocharging the VR6 wouldn't do anything for its fuel economy, but you'd have a stupidly fast car. Also, the base N/A 2.0L is cheaper to run than either of these powerplants.
Notice I highlighted the cost. When talking about fuel economy, that's the number that really matters, not gallons. So the turbo engine not only has to overcome the MPG numbers of its N/A counterpart, but it has to do so by a margin that justifies its more expensive fuel. This is one rare example where a turbocharged engine provides a lower cost of ownership than one of its N/A counterparts.
The Subaru Impreza has an N/A 2.5L and a 2.0T that both get 18/25 MPG. The turbo is more expensive to run because of its fuel requirement, but it also provides more power. If Subaru was aiming for efficiency rather than performance, it would have opted for a 1.5T or smaller. Subaru downsized by only 0.5L where VW took it down a whole liter.
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