Average for the TB is 1-3 mpg improvement...but if there is a vac leak or the tune not correct or the driving style (usually putting foot in it more) effects it all.
And yes, dont trust the DIC...do it the old fashion way so you know for sure. The TB changes airflow and you should do the fuse pull (and make sure your running premium) and check to see if you have the weak MAF (check APEX's or GM Parts house threads on it) as the extra airflow will strain the weak MAF.
