Your fuel consuption figures are not too bad. I have the under powered non turbo 316 and it gives 7.5km/ltr round town through an auto gear box. I am pretty heavy footed tho and it is better on the longer runs giving aprox 9km/ltr. (Sorry you Yanks are going to have to do the maths.)
Its good on the corners with the reduced weight of the smaller block. Its all about momentum....
Would be interesting to see your set up. I wonder how much more power you are getting than me. I ve read that it is a common misconception that turbos reduce the fuel consuption but you are getting more power. Maybe at the same power out-put a turbo'd engine uses less fuel but you are having to drive the turbo so maybe not.
Good luck with the new car and wellcome to UB!