As Thrush says - once your at cruising speed on the motorway you will not use any different amount of petrol either on high or low boost as you won't (or shouldn't be on boost then). But if you wanna accelerate quick then you want as much power as necessary so you run high boost.

As mentioned before only benefit of having switchable boost like this is for driving like around town where you don't want the boost to come in high and end up in someones boot.