I've been leveling a Pandaren priest in full heirloom gear, and I've found Discipline to be the strongest spec so far for leveling. I've taken Shadow as a secondary spec, but for the damage output it seems I do very comparable damage (maybe even more) in Disc, while maintaining high survivability. Also take into consideration that the 15% hit passive for Smite/Holy fire is given very early in leveling, you won't miss any offensive spells. I find Shadow takes a bit of ramp up to get damage out, and I haven't touched holy at all.
Is Disc meant to be the go-to leveling spec, or are Holy and Shadow supposed to be comparable and I'm just playing those specs wrong? Is there a point when Holy or Shadow become better specs for leveling?