As someone who lives in the area, the Bergen Beach branch is usually full of the people who live in this area until it hits the junction. It will often filter in passengers along flatbush all the way to downtown brooklyn and vise versa. The issue arise when there aren't as many Bergen Beach branch buses as there are Kings Plaza buses on the 41 and when it gets to the junction in the evening rush its crowded. So, its not much of an issue going northbound i.e to downtown BK, its the issue for when its coming back in the evening which isn't much reliable. I usually take it often and over the years it has gotten better but they do often short turn the bergen buses at prospect park.
What the B40 does to right to me is that for one, it separates the branches entirely by number, the times people who wanted to go to KP got on the wrong bus is funny. And second, its a rush route i.e a limited.
What I think it does wrong is it being cut back from downtown BK, but we have to keep in mind that the B41 is being planned to become an SBS in future, so this will really be a conduit avenue since there will be a local here along with an SBS that will go downtown, along with transfer connections to the and the trains that go in the same direction.
I think the B40 rush should probably go to at least Grand Army Plaza or the Brooklyn Museum, but it does seem to make sense to end it at prospect park.