But an overtaking aid shouldn't be judged in Km/h differential, much less in Km/h differential at top speed. It should be judged in time saved in the straight. A small speed differential while you are slow is as good as a large speed differential when you are fast (in terms of time saved), plus if you activated your DRS while slow, you'll still have it active when you are fast and drag limited.
We like to think in term of car B is xx meters behind car A, but in reality, what counts is that car B is xx tenths of a second behind car A. Time is the constant, and it will translate into different distances at different speeds, hence the accordion effect we see in every straight between two slow curves.