I think if you painted the ceiling and beams black, it could look quite nice..particularly with the grey walls..I've been debating what to do with the ceiling. I am thinking once i get the walls painted and they look good, then I'll decide whether or not to paint the ceiling.
If you elevate your L&R speakers up towards the centre of the screen, then having your centre speaker above the screen will be fine..Also something that I would like to do is move the center speaker above the screen, i'm thinking of making a platform to hang from the ceiling and put the speaker on (as you can tell, the wall behind the projector is a rock wall and not really suited for installing shelves or anything on). Then taking some acustically transparent fabric and running it across the top of the screen covering the center speaker. I was also thinking about doing the same for the two front speakers, moving them up center with the screen (however still visible).
Otherwise if you leave them on the floor, then I would mount the centre speaker below the screen..
The other thing of course would be to make a false screen wall and mount your speakers behind it..