The pic above shows the results from the different algorithms rendering the same scene (a bunch of stuff close together and overlapping, lit by the sun and a radial light). As you can see, the time saved by using the correct optimisation setting makes a substantial difference (about 20%) - even with a simple scene that renders quick. Note the excess ray hits and misses with the Grid setting.

With an animation or a scene that takes hours to render, 20% is not to be sneezed at (assuming a linear progression).

I had intended to post an example showing the benefit the other way, but I'm not sure I know what the engineers mean by a "uniform" scene. What I thought fitted the description resulted in an even bigger advantage for BSP, so either I've got the meaning wrong or the Grid setting is completely frelled.

God I'm sad.