It all depends on your setup. I'm sure some guys have pieced together a turbo setup that shot their mileage to hell. I also know of a guy here in CO with a built and boosted b-series EG capable of close to 700hp when set up for the track, but he got killer mileage on his street setup because he never even got the turbo spinning.

Also, my buddies drive hopped-up saabs that make around 250-300whp and 350+ wtq but get close to 40mpg when they're babying it on the highway (partially by virtue of tall gearing).

I don't think there is any formula for success, and I also don't think adding a trubo system to your car with the goal of saving gas is remotely practical, or even likely to work. Think about it this way: you spend $1,000 piecing together your system, and if you're lucky it might save you a couple gallons a month (if you're unlucky it could cost you much more). It would take an awful long time to recover $1,000 at a rate of $8/month. h: