I am pretty sure bonuses like chance to get item rarity and bonus gold on NPC equipment is not used, since there is specific stats for those things that do not go up when you equip them on NPC in your party.
But what about item bonuses like "bonus exp per kill", do the NPC in your party benefit from that? What item bonuses do they benefit from?
Do you split your experience with your party members? Do quest npc take any experience from the group?