I don't know? I assume less than 40-45% increase in overall damage since you're going from 300% to 420%, which is 40% more, but its also with a ranged damage buff on the hat as well I think. You could have done this math as well, you know.
The 30% buff to severe rainstorm is more useful (100% -> 130%) since severe rainstorm already does decent damage and is an AoE and isn't relying on a gear that gives it +200% just to be decent. Metallic Sound has the same prob now, as to increase its damage by 25% you'd need a gear that does +100%.
Arrow Vulcan is a skill that I have for fun (remember my old time 99 with op). The animation is very beautiful, so I won't make a int char to use mt while it's a 4fun and ugly as.compared with av... maybe 10-15 years to boost it... I wait until the end..