floorstand speaker B W N 804 as center

No experience with B&W, but I have used a vertical center channel before. Personally I liked it alot. It provided extremely even sound across the front three channels. If you can swing the same speakers for the rears you will be even more impressed. A vertical speaker might force you to elevate your screen higher, but even a horizontal speaker arrangement will put the center above or below the screen. Keeping the tweeter of all the speakers at the same height and of the same type is the key, or so I've been told. An article I read kind of backed this up by saying that the human ear can more easily locate higher frequency sounds. Hope this helps.

yeah man do it,I use seven keltics all round in thx ex and dts es.The small B&W centre speaker is a better match for the 805 than the big one is for the 804,so I would either do the 805+one or two ASW4000 or 804 all round

I know of people who use the exact same speaker for the entire surround sound field. They all swear by it. So give it a try.

Go with the vertical center channel by all means.It will sound better.Even if you have horizontal center now,set it on its end temporarily and sport a listen,I think you will like what you hear.