So no one wants cooldowns on abilities and globally increasing cost per cast hurts solo/small scale much more than group play. Large coordinated groups can get kills on even tanky players in seconds just by focusing attention so they can afford to give up damage in their builds and go heavy enough on sustain.
If you want to improve performance by disincentivizing large groups spamming abilities without penalizing everyone else why not use Battle Spirit to add a ramping cost increase per group member to AOE abilities? 1% per member would barely register for groups of 2-6 but once you got up into full size groups it would be a serious penalty and would help balance out the massive advantage bigger groups have.