The other issue is the sound from the two drivers underneath the speaker. If the speaker is flush to the wall, you will effectively lose half of your sound!
Probably not half but some. Depends on where the crossover is to the front speakers, I am not familiar with the Beolabs in question. Might be able to compensate by setting the crossover freq for the surround processor for the center higher.
There are two drivers on the underside of the cabinet halfway between the front and back. If the speaker was flush mounted in the wall, these drivers would be behind the wall.