When I was completely stock I got the same exact average in the city as I did after I put on my Magnaflows, 12-13 mpg, and best of 22.1 mpg during a 120 mile round trip. Keep in mind I've got 3.55's stock.
After putting on my intake, and bama performance 91 tune, I'm getting 16-17 mpg on average in the city and running the car way harder than I was before 24/7, and I've gotten a best of 22.3 mpg on the same trip on the highway.
I always saw guys here saying they got 19 or so in the city, but I could never get near that before. I also remember two guys here that had completely stock s197 GT's and were getting 8 or so mpg city, and they said they both drove in normal manner.