During the time that you back off at high rpm to induce the bang MPG will decrease, the rest of the time though it wont be effected.
The cells on the map that are changed arent generally ones you are likely to be at much when driving with economy in mind, and the amount of fuel involved is very small, it doesnt take much petrol to make a small bang.
No useful performance effect, unless you happen to have a turbo you are trying to keep spooled, its essentially a very mild version of what happens during antilag.