That turbo kills mpg anytime you get in the boost. I have 3 turbo and I get maybe 15-20 (got a heavy foot). If I am careful, I get 25-30 easily, but then I get bored and it involves never passing, and taking 30 secs to reach 30mph. Pointless to have a turbo at rhat point.
Personally, I think if you get the turbo, you should try to get as worse mileage as possible. The more I spend, the more fun I had ;)