Hmm. I was overtaking someone at 80 when my battery was at 5%, and the car was flying. It was at 4% when I stopped by the charger a couple of miles later. The last mile was in a 20 mph zone so I can't say for sure I wasn't limited, but I wasn't conscious of anything.
The car did ask me, at about 15% I think, whether I wanted to go into power-saving mode, but I knew I didn't have that far to go and said no. The car was absolutely fine for the next 20 miles, as I said overtaking at 80 a couple of miles out of the village. There were some alerts on the dashboard when I stopped at the charger, and I think I heard a plaintive "whew!" from the car at that point, but no loss of power.
I should say though that this happened in summer, in warm weather.