well, havoc states strictly single target spells so.. I guess not
anyhow..is it 15 demonfire bolts total, or 15 bolts per random immolated target. tooltip still doesn`t say for sure^^
if its the latter.. I don`t think it necessarily is op. just look at boomkins collapsing stars.. if one manage to hold it up for 10 sec. that's 10k SP worth of damage