I'm running 5, and although I haven't PvPd yet, I did do some dailes and mobs dropped like prom dresses. I usually kill chillmaw and the 3 cultists in about ~10 secs, this morning chillmaw dropped before the cultists even hit the ground. Five earthquakes later and they were dead.

I did notice some very annoying lag, so I went into my video settings. Seems like Bliz defaulted everything to "fair" on settings (or maybe it was "good"). Anyhoo, dropped all my slaves to "low", and got rid of that.