any economical gain will be thwarted by the cost of the set-up. Not sure how it would gain MPG because when the engine is not under load there will be no boost. I think this is why new cars use turbos. Because they can make good H/P with small cubic inches, but when you step into it and the boost hits the H/P goes up. Under normal driving conditions the engine maintains it's good mpg. I'm guessing if you drove around like a mad man and stayed in the boost your mpg would suffer.
But I'm probably wrong about all that.