The turbo does extract energy out of the exhaust, however, it's not without a penalty. With the increased backpressure from the turbo, the engine must consume additional fuel to compensate.

The energy that is extracted is utilized to compress the incoming charge air, with the expectation that additional fuel is delivered, and therefore the engine produces additional horsepower.

If the additional horsepower is not required or desired (steady state cruise), you'd be hard pressed to make the argument that the turbo would increase fuel economy simply by raising the temperature of the incoming charge air.

Remember, the additional fuel economy can only be obtained if the initial loss generated by the turbo's backpressure is overcome. I'm a bit doubtful that this can occur simply by elevating the temperature of the charge.