You gain max XP by running the highest GR you can run considerably fast and consistently (3-5 minute clears). Bonus XP gear has been nerfed to death and is mostly useless, with some exceptions but those don't apply here. Just gear for max DPS (or max survivability and buffs, if you're the support character in a group, which is what your Cain's character and the guide you linked is about).
And a general note: Quin has sadly left Diablo 3 and is just playing WoW these days, his guides are outdated for the most part as he hasn't touched the game nor his guides in about a year, and some stuff has changed since then. So look for other builds in the build tool and always make sure they're from this current patch.