"Hey there. I think I've identified your problem.
If that is your opener, and you use no more additional spells then that, I'm fairly sure that ember regeneration is your main issue. Since you're haste stacked, your mana regen should be so high that you're able to keep a 100% uptime on Rain of Fire. That'll heavily impact your ember regeneration, and should in turn impact your dps as well. In 5.0-5.1, gosup used to do more damage with incinerates than chaos bolts, but that has turned around now. Just try to focus on maximising your ember regeneration, and chaos bolting during procs and DS. Also, try to cast a second Immolate between some of the Incinerates, just so that you can get the buffs onto that as well (preferrably after the Pandemic mark
In a multi-target scenario with adds that live long, using Havoc on an off-target and casting Immolate then 2 Fel Flames will yield the highest ember return per execute time. Of course, this depends a little on your effective crit chance, but should long term give more embers than using 2 conflags and incinerates. This is especially true for fights like Megaera, where the damage only counts on one head - you still want to keep immolate on both, just for the sake of embers."