It's stated that Sanji's passion burns hotter than fire. So as long as Sanji keep going, the fire intensity could increase. But, Sanji needed some kind of durability or defense that can keep his legs from getting burned. That's where Exoskeleton and CoA come in play. Now, you could say that his CoA is not strong enough to keep his legs so he needed Exo, but Sanji's DJ is hot from the start.
Basically in comparison:
Diable Jambe - orange flame - Basic CoA
Ifrit Jambe - blue flame - Adv CoA/Ryou
Surtr Jambe - black flame - Internal destruction haki
The stronger your armament, the stronger the durability. Sanji already got Exo, he just need to train his CoA more to increase his firepower. And I think Sanji's third stage Surtr will requires level 3 CoA.
Lmao, that would be dope no cap. Instead of Buddha's Palm, it will be Devil's Leg.
Love that movie btw.:cheers:
Red Line is always available.