I did a run down test on my 2013 FFE in the middle of the summer on a hot day and measured 16.4 kWh at about 45,000 miles, that's down from 16.7 kWh I measured last year in August. Then ran the test again a few weeks ago at 49,000 miles and got 15.9 kWh. My recent tests have lead my to the conclusion that ambient temperature has a significant impact on battery capacity with less capacity being available at colder temperatures.
The measured battery capacity of my 2017 FFE at 7,000 when I bought it last June was 28.8 kWh.