Apparently there are no diminishing returns with stacking multiple Augment Evokers in a group. This might be self-correcting for a 5-man dungeon but has been demonstrated to be horribly broken for a raid team as noted in the post linked below.
2 DKs are doing ~ 3 million dps because there are 22 Augvokers in stacked in the raid group and they are killing heroic boss encounters in 20-30 sec each! 