You definitely should beat the game first. Beating Pokémon X really isn't that hard, and it is super useful.
If you want to be a competitive battler, you must have access to the IV checker. He will tell you the overall IVs of your Pokémon and will also say the stats with the highest IVs. (They're perfect if he says they "can't be beat.")
There are other benefits to beating the game as well. For instance, there are some Pokémon and TMs that are only accessible on Victory Road.
Getting Mega Stones could be an issue too. Mega Stones for Absol, Mewtwo, Gardevoir, Alakazam, Banette, Garchomp, Gyarados, Kangaskhan, Manetric, Mawile, Medicham, Pinsir, Scizor, and Tyranitar must be collected post-game.
So you shouldn't stop where you are to look for a competitive team, since many elements aren't accessible until after you have beaten the game.
Hope I helped!