I get exactly the same mileage as my 135i; 16 mpg overall. I am either in bad traffic or when clear roads my foot is clearly planted WOT.
I have not had any long drives yet. But I have seen short times of mid 20s or so. Again just like my 135i. The 135i long drives I did see 26 mpg, which this ATS should see. But then I drive hwy hard too. Getting good gas mileage the way I drive has never been a strong suit of mine.
Another data point is when I had a Jeep Grand Cherokee with the hemi engine. I got 10 mpg! So I feel terrific with 16 mpg.