Wrong, magic resist gear, sockets (i.e. 4resist to all gem) reduce your chance to be hit by spells and on lv70 it has a 360 cap (saying excellent on your char tab). Spell penetration does the opposite of it, ignoring your targets magic resists (13 per gem ) . Note that there are 2 types spell penetration which works as i said and spell hit chance which is moslty used to pve and is calculated on %