With an exponent of , you basically get the same curve as the Uncharted 2 curve except for the really bright colors. The upper tail actually conserves more colors instead of abruptly cutting them off. In the actual shader you only need to calculate a single pow, so it's not that much slower than the one without the additional parameter.
As soon, as I can test it out, I'm going to create some comparison shots. Even though I can already tell you, that it should basically look the same, as the Uncharted 2 Tone Mapping operator, except for the fact that bright colors are converging more smoothly to pure white. The rest of the colors look as "filmic", as the Uncharted 2 operator does.
Have you some comparision shots (color gradient in linear,reinhard,uncharted and modified reinhard) ? This would be quite interesting.