You don't get any meaningful extra speed when you develop more.

You get extra contrast, which may make your results more usable.

Remember, the film's "speed" is essentially a measure of the film's ability to render detail in the shadows, and is mostly independent of the film's highlight behaviour.

Expansion and contraction are used to adjust the contrast (gamma) and the resultant rendering of highlights but, in the most part, they don't affect whether or not shadow detail is captured.