I wouldnt mind a hp increase to bosses or any other adjustment of some sort if they let each roles have their own limit bars.

2 tanks contribute to their own limit break bar and decide on their own when to use it
4 dps contribute to their own limit break bar and decide between the 4 of them when and who to use it
2 healers contribute to their own limit break bar and decide between themselves on when to use it...totalling into 3 separate limit breaks.