So wait. 25% with an XI is terrible, but at the same time you can't expect a buff/DD hybrid to be able to provide the same level of buff as a buff job that's designed to be a mage? Isn't that kinda the point then? 25% isn't as high as BRD can hit without Marcato let alone with, it's not going to cap your haste even with the spell Haste, and it's not a guarantee, thus it doesn't provide the same level of buff as a BRD. That said it's not terrible either because it does serve a purpose and gives COR a better ability to actually buff.
Since Delve went all 6-man and everything I've not actually seen a COR in endgame that I can think of. COR is like GEO, but it's even worse off. It's a buffer you take when you're doing something more than 6-man content because you can afford the extra spot. COR for instance used to be taken to Tojil runs because you had room in the party for a COR, now that it's filled with a SCH you're not taking a COR unless someone is in a different party, and if that happens it's probably the SCH which means BRD swaps, which means more work than most people care to do. GEO can fill this role from a second party at least, debuffing as opposed to buffing and allowing the SCH to stay in the main party without any real downfall except MP limitations due to lack of ballads, an issue they can live through though, CORs on the other hand are limited to a single party.
The idea here isn't for COR to be a replacement for BRD, but more of a job you can take instead of BRD that actually works well. Even if COR wouldn't be as powerful as even a standard BRD it'd be powerful enough to be a viable alternative.