It's not like it will break the game if Chocobos in the overworld can actually contribute as a healer or tank at the same-ish level they did in ARR. My poor choco just wants to keep me alive in FATEs! Tank mode choco can't even strip hate from a healer just spamming filler GCDs.
Just tune up whatever the scaling algorithm is used specifically for level 51-100, or add (hidden?) chocobo traits that update their potencies at each level cap (60, 70, 80, 90, 100).