Single target dmg is a problem, yes. The sharp drop in aoe if a tank moves the mobs is also not the best of designs. I'd much rather the MoP/WoD aoe rotation (a toggle that makes your single target spells aoe, but cost soul shards and do around 50% of regular damage, but if there are enough targets you gain more shards then you lose. There's complexity in both sustaining soul shards and maximizing aoe. Also Rain of Fire no cost, but doesn't overlap, and does more damage so you can reposition it whenever). Much funner then "cata and demonfire/incinerate spam and rain of fire". Tuning can happen so dmg is good, not op.
As for single target, a simple solution might be reducing infernal soulshard gain by 50%, so its twice as weak, but increasing all single target damage (non aoe incinerate, chaos bolt and conflagrate) by around 20%. Less powerful cd, but stronger sustain single target.