It apparently is, just barely.
One thing you gotta remember is that Flamethrower straight up stops your Auto attacks and based on my test, it's a 50 potency attack every 2.5 seconds.

They're pretty much equal if I understand well, if you use Flamethrower you'll get a 10s increase but after that your DPS will be lower than an AoE rotation with no Flamethrower, because you didn't generated heat during that time.