I'd have to agree that having the /butcher command run every couple of hours, even if it's just to redistribute the mobs across the map instead of having them focused in certain areas, would probably be a very good thing. It would automatically solve the spawners that are out of control, as well as possibly help with the lack of slimes. I agree that it wouldn't be a complete solution, but since it could be done probably with a simple linux script it couldn't hurt to do it on a trial basis to try it out.