I am against the majority here, but my car ran 12.89 when it was brand new and totally stock. It has since run the time in my sig, also totally stock. Negative D/A under ideal conditions.
When I load the tunes it drastically changes the full throttle shifts, barking second hard and often chirping third gear also. But not a tick faster. Even with extensive tweaking my tuned car is not faster than untuned.
But it is much more responsive, less throttle lag, cooler operation due to reprogrammed fans, etc. I especially like the way the stability control is 'pushed back' so it rarely intervenes.