Xinerama is not what you want to be using. Using Xinerama means that you can't get graphics acceleration, and it makes sense that KDE is preventing you from enabling compositing for this very reason.
Use xrandr instead.
As for the open source driver; why not try it? I use the open source driver, but I do not use compositing (running a bare bones Openbox setup) so I cannot report on its effectiveness here. (You should be able to use xrandr in exactly the same way with Catalyst and with xf86-video-ati. However, you may need to increase the virtual screen size with Catalyst, since it requires that kernel mode-setting be disabled. Instructions are provided in the thinkwiki article I linked to above.)


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