When I turbo'd my car, I actually imporved my gas mileage. But that is mostly due to the fact that I tuned my car once becoming turbo. I daily drive my car every chance I get, but it is undoubtly down more often than it would have been had I left it NA.

Passing emmisions also became more dificult. The first year, I sat in the car while it was on the dyno (or whatever they call it) and tuned it to 15.0 (with an SAFC) to make it pass. After that, I upgraded turbos again and didn't even want to deal with it, so I registered my car out of the emissions area.

Now my car gets even better gas mileage (assuming I'm not too heavy on the throttle) because I have an even better fuel managment system. Right now My Maftpro will self adjust while cruising (as well as at WOT) to whatever AFR I tell it, so I usually cruise around with a 16.0 AFR. When I get on it, that mileage goes to shit.. but thats pretty obvious!