What I would recommend is don't nerf the chocobo xp gain. Instead improve the onion situation by making onion seeds give more than one onion per seed and have them grow much faster. It would also help to give Thavnairian Onions a direct loop. Then it would be much easier to get onions for birds. (Note that my bird has been 20 since ARR so this is just to help out those still working on birds.)