Originally Posted by Kal Rubinson
I take a different view. If all the speakers are, now, in the ceiling, adding a real center, either as a standalone or, even, in wall will help to bring voices down and help anchor them to the images. I would be less concerned with timbre matching than I would with convincing dialog.
Well, I wouldn't say that my view necessarily differs from yours because I totally agree with everything you said regarding this case. I was just explaining, in general, why most center speakers tend to be different from the others. But the OP seems to want in-ceiling speakers for whatever reasons, even acknowledging the fact that this is not optimal, so that's the assumption I worked from. If he would be willing to compromise with a center speaker in the room or in the wall near the TV, however, then I'd throw my support in for that, sure.
In fact, if you want my real view, I think that in-ceiling speakers are a huge compromise and that the OP should seriously reconsider going that route, regardless of whatever wiring might already be in place.