Souleater : "Does damage to the enemy at the expense of your own hit points."
I'm pretty sure it only applies to melee attacks. And I don't think subbing DRK just so you could use the ability (lvl 60 RNG) would be worth it, many other jobs offer much better compliments. Unless you're talking DRK/RNG, in which case I'm sure there are much better jobs that compliment it, and you wouldn't be able to use sharpshot.
And you get barrage at lvl 30 RNG, so you'd have to be a 65 or higher (I think) DRK.
Good idea in theory, but as far as levels go and subbing a job just for one ability, it really isn't practical.
And it's not like RNGs need to be doing any more damage than they are.