I think that is incredibly optimistic for a production vehicle. I think it is likely that production vehicles with setups like this will have one 350 Watt panel on the roof. A typical house roofmount of the same would produce ~1.5 KWh per day.

Car environments tend to be worse than that due to the angle and likely regular proximity of even mild shading. Its certainly enough to eliminate vampire drain, which is helpful at times, but I don't see how it will add an appreciable amount of mileage otherwise.