I don't run any spell pen on my 4x mages for PvP. Casters are so ineffective against me anyway I don't target them until everything else is dead and most non-casters don't get any resistances above the 75 for MotW if they have it. Are shaman seriously having trouble bursting people down because of spell pen?

Granted I have a bit more of a survivability challenge being mages so I don't socket damage. I'd go either 70-80 spell pen or 0.