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One thing to keep in mind: a turbo can be designed for performance, or for fuel economy. Although a turbo CAN up FE, sports-tuned turbo setups frequently do worse than NA engines of the same HP rating.
Most of it comes down to turbine sizing. Suppose you need more top-gear HP at freeway speeds: you shoild size the turbine such that full boost comes at.WOT and freeway RPM without opening the wastegate.
)The downside is that this would use a "huge" turbine that added little power down low, and was laggy as hell, due to the massive turbine inertia.
Typically, turbo engines modified for aviation use (steady-state power) will use a 7L turbine for somethi.g like a 2L subaru.
Cases in,point: my stepdad owned a mid-80s Saab 9000 turbo. It was known for high mpg-but was very laggy. The 2002 WRX I owned had lots of power, virtually no lag-and got 17 Mp
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