If you run in full screen, you can - indeed - not go offscreen with your cursor. Running in windowed mode is the only way to solve this as far as I know. And yes, windowed mode means performance decrease, there's no way around that either. Sorry for the bad news
However, with maximizer(and thus with keyclone), you can make the borders of the windows disappear, so they fit onto the whole screen. But even in this case, the performance decrease is still going to be there.
You could limit the fps (maxfps/maxfpsbk in your wow config files or through keyclone) of your clients to gain performance on your main client.

p.s. We have a separate forum for support topics.