[Quitti]

Yeah, penetration bonus items stack. That's Void eye + spell focus + rune smasher = "cheap" +5 penetration iirc. Then there are RoW and RoS and probably some artifacts that also give penetration if you have third misc slot for the caster. And like you implied, more magic skill gives penetration (+1 penetration for every 2 skill over spell path requirement). At least that's how I remember it.