Garchomp is going to be OU for sure, ask any OU council member and he will confirm it. He just isn't yet, because they are thinking how to bring him down (Sand Veil ban, Sand Veil + SandStream ban, or something else). CB Garchomp is a very good poke, and a perfect check to Jirachi. He ohkoes any 252 HP Jirachi easily, while being able to switch into anything that he carries (Iron head, Psyshock, Thunder, Flash Cannon) except from Body Slam and Water Pulse (you must get really unlucky, as Water Pulse has only a 40% chance to confuse you, and then you only have a 50% chance to attack, so there is only a 20% chance to get confused and be unable to attack).
Part of the OU QC team, message
me for a check!