I would be disappointed if the mera didn't produce the hottest flames.
I mean the reason Sanji can even use a hotter flame is because his body can withstand a higher temp now, but mera users are logia so that doesn't even matter. It's literally called the flame flame fruit and it's being outdone by a fruitless genetically modified cook, kinda stupid if you think about it. I know heat isn't everything but c'mon should a flame fruit user not have the ability to control the temperature of his flames? I guess the cope rn is that it's quantity vs quality, and the sheer quantity that the mera can produce casually is unmatched.
Sanji rn may already have the hottest flames (def debatable) and it stands to reason if he improves his armament to shield himself better he can go even higher, so it's likely he ends up with the hottest flames by EOS.