Heat has no real upper limit until the universe dies in what is literally called 'heat death'. Its one of the reasons Ace 's fire losing to Akainu's lava pissed me off in One Piece. Lava has a heat limit, since the rock will eventually disappear. Fire has no such limit.
Though you also have to deal with how temperature affects other things. Like heating up the air enough to scorch your lungs. And if you're immune to your own powers, I doubt your clothing is, lol. Or the ground at your feet. The building around you... Turning the heat up too high could kill the user just as easily as the opponent.
As a side note... How are his sleeves fine after the metal reached melting temp?