I've gotten 100%, but it wasn't for efficient driving. The app isn't smart enough to recognize when you're driving in REx mode. So, if you're on a long trip and have to stop for gas to make it home, the final leg of the trip is viewed as if you used nearly zero charge. Thus, it thinks you've driven 50mi on <1% of the battery.
BTW, it might be worthwhile if the picture was actually something different, but if you look, the short animation at the beginning of the efficiency rating shows the full range of "growth" before settling back down to whatever your score is, and that's all you see when you score 100%.