Firestorm: bigger AOE is a mixed blessing?

Does anyone here use firestorm and keep it a smaller AOE? I used to think that the larger the AoE the better, but I'm now finding that it's almost impossible to kill single targets with it (+40% AOE from passives).

I used to run with improved aoe passives, support, added lightning, and elemental proliferation to get everything good and shocked. Then use some sort of fireball for dps.

If you're trying to firestorm single targets though, improved AOE is probably not going to work well.