For explorers guilds, there are no additional benefits beyond 1st level. Having an explorer guild confers teh following bonuses (general who ends his turn in settlement can gain a +movement trait) generals more than x moves away from capital can gain some unique anciliaries like vasco da gamma and marco polo. That's it, pretty crappy if you ask me. I usually get a basic one if it's offered for a chance at the unique anciliarries, and thats about it.