Speaker on its side as a center?

I am curerntly using a B&W 804 as a center with 2x 804's as mains. 2x 805's as rear. The center 804 was one of a pair, but I lost one courtesy of the shipper. I would like to lay the center 804 on its side under the plasma TV. Idea is to have this on some kind of a shelf structure under the TV platform. Any thoughts on performance of a speaker on its side? thanks.