I went back through some videos I shot when I first got the car, because I knew that I did some of them at relatively low power levels. I have a test drive where I was at 50 miles rated range left and did not experience the top end power limitation despite doing 5 or 6 insane launches in a row. At that time, I didn't have the ability to capture telemetry data but it was my subjective impression that the launches were slower than the ones I experienced with a fuller battery pack, but there was no visible power limitation. I know that I have seen it at least once and that was on a trip home where I was concerned about making it, so I never attempted to accelerate hard enough to test it, but I remember seeing the line at that time. It was quite cold that day.
I'm just under 50% now (after not charging overnight) and not seeing it despite spirited driving.