[pgfpro]

This might help answer your question.

I have installed around 10 turbo systems on N/A Honda engines. I did not change anything on the engines. Completely stock. These were just bolt on turbo kits and before the larger injectors, stronger heaver clutch and engine management were added the engines got the same fuel mileage as they did N/A at light load and freeway driving. The exhaust back pressure did not hurt fuel mileage at these loads. In fact I measure on two different engines that the turbo back pressure decreased at light load verses N/A because the exhaust system pipe diameter increased from 1.5 to 2.5". So there is no reason to install a exhaust cut out to bypass the turbo at light load.

As others have already said when you see two types of engines of the same family one N/A verses one turbo there's a reason the turbo engine gets poorer fuel mileage.
Lower compression.
Heavy duty internals and drivetrain two handle the HP at high load.
Fuel enrichment to keep detonation suppressed because of the uneducated car buyer.
etc.