Originally Posted by
Biomega
Do *NOT* take current Mythic dungeons as a benchmark. They're tuned for what, 685 gear? And people do them in 700-730. It's only natural that absurd things happen when you so vastly overgear the content. I've done runs where no boss was alive for more than 15-20 seconds, that makes any sort of conclusion you would draw from the resulting meters almost completely unusable.
Mythic+ will be a whole different slice of cake. Fights will take a while at high levels, giving Fire a chance to fully unfold. Also, as was mentioned, it will be much easier to use Combustion on trash if you know you have enough time for it to come back up for the boss.