The quoted gains for city driving are up to 10%, which would take you from 15 to 16.5
It would be my understanding that this kind of gain is seen in the EPA test protocol, not in the real world. Actual gains will depend greatly on driving conditions, driver choices (auto s/s on or off), use of remote start, etc, and are likely much less than this.
The greater benefit is zero emissions during idle which I personally think is important
Some think this important, others do not. I have been a serious student of this topic for many years, and, as a result, put myself in the latter category. Data I have seen show that motor vehicles worldwide account for a small fraction of total CO2 emissions. Other vehicle emissions have been greatly reduced over the years and continue to be reduced. Not a significant issue IMHO. This is a cost/benefit issue. Is the improvement in emissions worth it?