There isn't a real straight forward way to answer this, but in general adding a turbocharger to an engine will decrease fuel economy. This is probably mostly due to people using the additional power they provide. However, turbocharged engines generally have lower compression ratios (which in general means its less efficient), and they also run rich when boosting which also lowers efficiency.
The fuel efficiency benefit of turbocharging mainly comes from downsizing the engine. So, for instance you take the Mirage's 1.2L and replace it with a 1.0L turbocharged engine. Now, theoretically you can have the fuel efficiency of the smaller 1.0L, but the power of the 1.2L engine. This is just a rough example.
View my fuel log 2015 Mirage ES 1.2 automatic: 49.0 mpg (US) ... 20.8 km/L ... 4.8 L/100 km ... 58.8 mpg (Imp)