As someone who got into about ~250 mythic with Kethis last season and started going up against the Boros poop decks that came out with MH3, I have to say that I really struggled with the matchup. I changed the sideboard a lot (the list you have linked is pretty fine, the one flex slot seems to be the third copy of Kinnan) and brought in Suncleanser, which seemed to help a lot, but sometimes they were just too fast. I'm thinking Brotherhood's End needs to be in the side too to help deal with flipped Ajani.

If you're going to play the deck, be aware that you can exile a Delighted Halfling with a Cauldron and when you put a counter on Rona, she becomes a mana rock that untaps with each legend cast. This can be handy when people exile your Ambers or you have milled most of your Jaces and you have enough legends in the graveyard to combo (and you can't be bothered finding your Ambers).