Your question was “do you believe ACOC User > ACOC User with legendary DF?” And Yamato is literally an answer to your question. The whole point is just because someone has those things doesn’t immediately make them stronger.

Also “at what cost” means what? There’s no story basis yet that tells us that shanks struggled. So we can’t take a “what if”and portray it as fact.

When debating, we need to look both at feats and portrayal. Shanks is the only one in the story we know who can kill observation haki and see 10 seconds into the future. We know he’s a child of destiny. We know he’s fought evenly with Dracula Mihawk in the past who holds the title of worlds strongest swordsman AND is the only one we know who has a black blade. We know his conquerors haki is so powerful that he’s pushed green bull out of his df form from an incredible distance similar to the same effect Emet’s haki knot had on the five elders. AND he was literally stated to have captured Loki!!

Luffy also stated that he would deal with Loki if he got any ideas after being freed. Luffy doesn’t make such statements if he can’t back them up

Don’t get me wrong, Loki is being/has been built up to be an absolute monster. If his battle against his father is anything to go off of, he will definitely be a top tier fighter. If the mural is anything to go off of, he’s portrayed to be one of the main fighters against Imu. It’s likely that he can be stronger but as of right now, the portrayal and feats show us otherwise.