I definitely prefer maxing spitfire at level 8, because you usually get Broken Myth by then, and your spitfire scales EXTREMELY well with it. You may not realize it, but your damage really comes from the broken myth stacks. Your goop scales well with BM too, because it increases the goop damage by over 100! The dragons breath however does not do much with BM, so it isn't as good as the spitfire overdrive. Also, when you mentioned the lifesteal over time with the AOE, by the time you use dragons breath, your lifesteal bonus from the passive is already gone! You have already used up most or all of your passive.

On a side note, you don't do a lot of damage in fights until you hit 6 BM stacks.

Nice picture but besides that, nothing really so far.