Up to 4k now on my ST and have seen no increase in fuel economy due to miles, only varies with speed/driving habits. I keep detailed track on a spreadsheet so any increase would be apparent.
Average is 25.14 with 72% highway. Highway 27-31 depending on speed, Houston city(lots of freeway) 24-26 Pure city stop and go netted me 21-22.
Seems to be following the EPA specs faithfully. I do hope there is a magic bump at some point, but I'm not expecting one.