i think it depends largely on the way u drive it, if u mainly cruse then ur car should have the same or even better fuel economy with the turbo back because as it will require less throtle to keep the car moving but if u give it a bit then it will have better flow in which case it will require more fuel to match the afr. but a turbo back exhaust is the very first mod u should do. u should never boost ur car higher with a stock exhaust and/or stock intake as you are working the turbo harder and creating extra heat.