I did an experiment connecting an older projector to my BV7-40 mkIII. I did it in the same room projecting to the opposite wall of the tv.
I found that you indeed need a second center speaker. I was hoping the beosystem 3 could mix the centerchannel into the two frontchannels, but I did not seem possible.
I too experience problems with the extreme dynamic range of movies. It makes it difficult to hear dialogue without turning up the volume. In another room I have a non B&O 7.1 setup with an Onkyo Receiver. The onkyo has a setting for limiting the dynamic range so calm scenes are louder and the level of action scenes are lowered. It works very well.
I'd love to see a feature like this in the Beosystem.