Hereās my calculations for final damage with BiS gear, lvl6 green gems, optimized stat allocation, and blessing+sacrament buffs (very easy to obtain)
Youāre looking at a 6.6k total damage difference total between each Barrage burst for each spec. Since barrage has a 17 cd and takes a few seconds to cast, letās say that you can more or less cast 3 barrages every 20 seconds. What that translates into is an overall ~330 dps increase, with maybe a 5% margin of error.
Conclusion: Investing 2 ranks and 10 skill points into a skill to get roughly a 300 dps increase is not good, especially considering your magic arrow alone is generating about 12320 Damage per second with the same gear/stats, it is completely dwarfed in comparison.
I still donāt understand why Barrage 15 is even brought up as a talking point, the damage increase from Barrage 5 to Barrage 15 is less than 1% of your total damage in your dps rotation. You probably get more damage by buying a single attribute level in Magic Arrow.
PS: Do note Iām just using a test dummy with 0 crit resist 0 defense as simply a reference point, the defensive values of an enemy just lowers the floor of all the values, and no monster in this game getās past 457 defense or 100 crit resist anyways.