It's a very solid team, but is extremely weak to Ho-Oh, and pretty weak to Darkrai. You might want to give Xerneas Rock Slide over Grass Knot, because Grass Knot doesn't really work as well as Low Kick does in Ubers.
Something like this would patch up those weaknesses.
Megahorn allows you to nail Darkrai without having to lower your defenses, as well as allowing you to smash Psychic types like Mega-Mewtwo Y. The second set can help you to completely beat Darkrai.
Your team is also very weak to Primal Groudon.
Try this on Rayquaza.
This set makes dealing with Ho-Oh and Primal Groudon much, much easier. Personally, this is my favorite set fir Rayquaza. You can destroy one of the great walls like Giratina, nail Aegislash and other steels with Earthquake, and you have the very useful E-Speed. Aqua Tail lets you ohko Primal Groudon easily. The fire coverage with V-Create isn't really necessary, as fire coverage is less needed in Ubers unlike in OU.
Hope this helps! :]