OK, here's a post with personal experience!
I was in very similar situation (WAF) when we rearranged our living area. There is a thread where Alan commented. The net was that horizontal is not ideal, but can be done. You want the tweeters on the "inside" (facing inwards).
As to performance, you know what? It's not bad. With some tweaking (toe in, angling downward, etc.) I've got a very wide sweet spot, and it sounds very good. Did I prefer them when they were vertical? Yes. But it wasn't as big of a trade-off as I had feared.
The speakers are about 13 ft apart, and I am sitting 16 feet back. Every position on the sectional sofa gets excellent sound.
You can PM me if you have more questions/concerns.
Rich
