I tried this recently as well. Soul Barbs just costs so much energy. I've thought about maybe hunting as a Curse Necro with a Mesmer. But still, the recharge time on Soul Barbs doesn't justify it either. Maybe using it on Blaze or Ghast would work since they have heals and might make it worth to bring them down using Soul Barbs + Curses & Hexes.
The only real target worth using soul barbs on would be frenzied charr warriors. Even so, I wouldn't run high ranks in curses for it which only enforces that. And yeah, its not really worth bringing a mesmer with more hexes as empathy/backfire would do all the work. You could have them run conjure phantasm spam for the lols but its not really doing enough for soul barbs to all a sudden be amazing. Even if you ran curses up to r14 then you would just be spamming 10e hexes on a single target to kill it, kinda pointless.
I did the math and at r8 curses, soul barbs on a frenzied charr while utilizing faintheartedness will deal more damage than using 2x vampiric gaze for the same energy.
As I am using life siphon regardless, I can exclude its energy cost but also figure in its soul barbs damage for soul barbs + faintheartedness;
23x2 (sb life siphon on frenzy charr) + 23x2 (sb faintheartedness on frenzy charr) = 92 damage 4 (2 degeneration faintheartedness) x 10 seconds = 40 damage
So r8 Curses, Soul Barbs + (Life Siphon) Faintheartedness adds 132 damage for 20e If I casted Vampiric Gaze twice at r14 Blood Magic, 57 x 2 or 114 damage for 20e
One interesting thing I found is that if a charr warrior receives zero healing after getting Soul Barbs/Life Siphon/Faintheartedness cast on them, they will bleed out and die without having to do anything more to them.