Well, I don't believe that Veteran level characters can throw hands at each other for 5 days straight imo.
Augur stated that Ace was pretty good even without his DF. He even emphasizes on how devil fruit users would be useless and weak without their DF but not the case with Ace here.
You can still be DF reliant and still be strong without strong haki. Look at Whitebeard, Marco and Admirals for example.
In the Ace novel, it's stated that Ace awakened CoA during his fight against Vice Admiral Draw who is immune to Ace's fire. And Ace's CoA was stronger than Draw's CoA even though he just awakens it.
Shanks also noted that Ace's haki was so strong to the point of Ace stepping on a snowy island, the snow started to melts. That's pretty insane if you ask me.