The last few levels to Rank 10 scaled up adding quite a bit of xp needed TNL. I imagine by Rank 19 you must need like 4 to 5 million xp to level up. I'm guessing there will be some sort of reward other than the additional barding down the line. Maybe a new achievement with item reward, or future perk for racing. Still, it's an insane amount of grinding for a bird that's really only going to be as good as a rank 10 chocobo, just with some added flexibility. Wouldn't surprise me to see them add the chocobo mask back in as a reward for hitting Rank 20.