I understand this concern.One downside to your ability, Karby, is that it takes away from the AF3+2 set ability, which enhances the duration of your party's buffs. By casting self-only (say Protect, Haste, Shell, Stoneskin, Phalanx, Spikes, Aquaveil) and then Aura-ing it, you might as well not even bother. Just as a note, it might end up making the set obsolete.
How about this, as an alternative?
Let the JA sacrafice our current Enhancement set, the full set of enhancements, to give to the party as a whole, effected by the same modifications to enhacement duration and composure that our gear normally would.
So say, while we have composure up, and we split our haste to the party, they would only get the normal duration haste if the RDM wasn't wearing their AF3. But with it on, it would get the same duraton bonuses it would normally get otherwise. (And RDM would just have to buff himself twice and just recast on himself.)
Working much in the same way Spirit Link does when you have mertis in Empathy.
This would prevent the workover issue of AF3+2 which, honestly doen't do all that much for us due to our limit of availability on single target buffs anyways. Refresh, Regen, and Haste, are the only things our AF3 enhances, and we've got a looong list of enhancing spells.
Or, alternatively (if the first ability seems overpowered), a JA that makes a buff we target on others "Aura" to nearby party members. Example: Just the JA, cast "Haste" on one of the DDs in AF+3. Suddenly, everyone in the melee party gets haste gets it for the duration it lasts on the target party member.