CharField max_length 2^n vs 2^n-1

You've mostly got it. It's not just the unique=True argument for 255, it's also that strings up to 255 long can sometimes be stored more efficiently.

So the answer is that there is maybe a point to doing it for 255 vs. 256, but for other lengths it's very likely to be pointless. Picking power-of-two lengths to begin with is often not done for a scientific reason (most of us haven't actually benchmarked that our 512-long field made our application run measurably faster than a 513-long field).

That said, there may be application-specific cases where the Django application is a frontend to a C application, where having N^2-1 strings is useful further down the line to efficiently store the additional terminating \0 byte.