There is a monumental difference between level 50 and 60 actually. The chocobo can be considered on par ilvl-wise to commonly crafted gear of that level. For level 50, that is NOT the ilvl 115 HW gear, but the ilvl 50 gear from ARR (The same applies to mob difficulty too, except level 50 mobs in HW areas ARE ilvl 115, while open world level 50 mobs in ARR are ilvl 50). At level 51, your chocobo will be ilvl 120.