In the recent Chapter Sanji used his new Ifrit Jambe and he explained it, by saying that with the Exoskeleton and his CoA his leg can now withstand much hotter flames. So he was able to create hotter flames, but would probably burn himself.
But we also know that Sanji said multiple times, that his passion burns hotter than fire.
With that i think Sanji's body can withstand as much hotter fire depending on his passion.
Meaning, if Sanji is not very passionate/angry in a fight, he isn't able to withstand very high heat and would need to use CoA to protect his leg - like with Ifrit Jambe.
But if Sanji is very passionate/angry in a fight, his body's limit can be broken and he can withstand much hotter flames.
And we have actualy already seen it in his fight with Judge:
In this scene Sanji used a hotter version of DJ without a problem seemingly - but the key thing is, he remembered his past and that he has nothing but hatred towards Judge. So he was very angry at that moment, allowing him to withstand hotter flames.
Now we don't know if he wouldn't be able to withstand those flames in a normal situation - without the angry aspect, but since he didn't used it against Queen, i would assume he can't.
Right now with Ifrit Jambe, Sanji doesn't seem to be as angry as he was when he faced his father, hence why he needed to protect his leg with CoA.
I thing that the protection of the exoskeleton and his CoA is needed for him to use hotter flames more consistently - having access to hotter flames even in ''normal'' situations where he isn't extremely passioned, so he can use them when ever he wants and doesn't have to get angry first, which is very situational.
Basicaly my point is, that his passion can break his body's limit and allow him to use hotter flames, but it's a very situational thing. As he said himself ''My passion burns hotter than any flame''.
Now with the Exoskeleton in combination with his CoA he has access to hotter flames when ever he wants, but it probably has a limit at some point.
But we also know that Sanji said multiple times, that his passion burns hotter than fire.
With that i think Sanji's body can withstand as much hotter fire depending on his passion.
Meaning, if Sanji is not very passionate/angry in a fight, he isn't able to withstand very high heat and would need to use CoA to protect his leg - like with Ifrit Jambe.
But if Sanji is very passionate/angry in a fight, his body's limit can be broken and he can withstand much hotter flames.
And we have actualy already seen it in his fight with Judge:
In this scene Sanji used a hotter version of DJ without a problem seemingly - but the key thing is, he remembered his past and that he has nothing but hatred towards Judge. So he was very angry at that moment, allowing him to withstand hotter flames.
Now we don't know if he wouldn't be able to withstand those flames in a normal situation - without the angry aspect, but since he didn't used it against Queen, i would assume he can't.
Right now with Ifrit Jambe, Sanji doesn't seem to be as angry as he was when he faced his father, hence why he needed to protect his leg with CoA.
I thing that the protection of the exoskeleton and his CoA is needed for him to use hotter flames more consistently - having access to hotter flames even in ''normal'' situations where he isn't extremely passioned, so he can use them when ever he wants and doesn't have to get angry first, which is very situational.
Basicaly my point is, that his passion can break his body's limit and allow him to use hotter flames, but it's a very situational thing. As he said himself ''My passion burns hotter than any flame''.
Now with the Exoskeleton in combination with his CoA he has access to hotter flames when ever he wants, but it probably has a limit at some point.
