I apologize for not researching more in depth, but if you allow me a few personal observations, I think we are talking here about two similar, but diametrically opposed evolved species. Boulengerochromis is a pursuit predator, endemic to Lake Tanganyika, Africa; slender, long body that gives it strength and speed for the chase.
Cichlas on the other hand, are chubby, lazy bums, ambush predators. Look at the large mouths that they have. The cavernous entrance of their digestive system is made for one purpose only: A LARGE GULP, to suck their pray in. Large, deep bodies gives them an initial push of speed once the pray is spotted, but nowhere near to the slender and pursuit designed body of the Boulengerochromis.
I know there's been a few sightings of Cichlas larger than what Boulengerochromis was thought to be, but I still think there's "emperors" out there that a Cichla can bow to

. I've observed wild Boulengerochromis in my tank, and upon feeding them live fish they would struck from across the tank ...catch speed and snatch their pray. In most of the videos of Cichlas in aquariums one can observe the "ambush" techniques.
Sorry for not being able to give a more scientific and compelling answer, but to date, I have yet to access some more data on their molecular structure. I'm new at this
Enjoy some of the pictures of my wild pair bud.