Don't underestimate the threat a tank can generate on this fight.

While doing training without any offensive buffs, I missed the first spore but managed to get a crit before the other fire mages (they were in first group to get spore so that's like 15 sec into the fight), with the result that the ignite was ticking for 3000-3500 (we were unbuffed afterall) with me getting threat for all of it since I owned it. If I recall correctly, I pulled aggro at around 160k damage done, with a displayed DPS of slightly beyond 2k. I may be remembering this wrong but I'm sure it was an impressive amount of time I could keep up that ignite before drawing aggro with only a minor head start for the tank. If we had had our best tank on the job (Thunderfury etc.), he had been using offensive buffs too (for what it's worth) and he had gotten a 90 second headstart before the ignite, I don't see an aggro pull happening within the next 90 seconds (or twice that), even with 5k ignite ticks, assuming you're Alliance.
I'm using 52% threat reduction.