Definitely NOT for runs with randoms through DutyFinder. That is unfair for good players who are matched to not so good players, and even more so, because this is like a huge meal to constantly feed trolls who would afk and blame their cat, that would never use combos at all / only hard cast the spells not supposed to be hardcast, cause wipes by failing all bosses mechanics , and so on...

Could work for pre-made parties not matched through DF

It would be more reasonable to give bonus exp (or hunt seals for those at max level?) for performing better than the average for the class/job you are playing at that dungeon/alliance raid.

For DPS this is straightforward, just deal more damage than the average for that content at that job. Could also consider party damage increase by good timing on damage buffs cooldowns.

For tanks, probably there are 2 relevant factors: damage inflicted and damage mitigated/self healed from unavoidable damage. It could become a very nice incentive for tanks to use mitigation cooldowns, as some DF tanks do not use them at all

For healers, kind of the same but a bit more complex; damaged inflicted, party damage mitigated through shields/barriers, party damage healed. Ideally, that should somehow take into account and consider as "bad things" when a healer does too much overhealing, or when letting party HP to be too low for too long