It ain't great. Matter of fact, yea, it's pretty awful. However, I never faceplant and the aoe output from fireball, rain and inferno ramp up the kill speed on max diff nicely.
Doesn't help that I have both Sent AT IO full sets in Flares and Fire Blast and a full Devastation set in Blaze. Attack chain is Blaze, Flares, FB, Flares, repeat. No hasten in the build.
I have no doubt I could reslot for better damage, take hasten, drop flares for blazing blast and improve the time considerably. But then I'd be annoyed by blazing blast's repel during normal combat like I was before his last respec.
Edit: Quick side note, if opportunity is up and I fire off flares and queue up fire blast, when both hit, I get both Offensive and Defensive Opportunity. Which amuses me.