You should really take it off the Boomkin and give it to the Fire Mage and Shadow Priest in that case. In every situation it is better to give it to a Shadow Priest of Fire Mage, as shown here:
http://www.mmo-champion.com/threads/...tent-The-Guide
The only way giving it to a Balance Druid is higher than a Shadow Priest or Fire Mage is in personal DPS gain to the Druid, and the difference between them is 9 DPS. So you have to decide if it's more important to give the Balance Druid 9 DPS more than the Mage and Priest would get, or more import to increase total raid DPS by 1k by giving it to the Shadow Priest.
For adds only ignoring single target DPS, gherkin is spot on as usual with Dark Intent matters