I love the idea, but i dont see the use for a double XP reward for well, any mission because in the end its just a number, good to level up but once in lvl 20 and dead its just meaningless, survivors may not benefit from this but then they go to that stonekin dungeon and get it in a day, so thats no excuse to complaign about not having double xp.
I think to make it worthwhile whilst not turning every mission into a 55, 600/smite or shadow form farmers fest is to increase the gold award thats already there and give something of a random item, could be consumables, sweet points, party points, or ale, or some % chance of dropping rare items (gold items, not elitetism stuff like voltaic spears)
Again i love the idea here, and it would encourage a lot of people to do the missions again, go pugging, have some fun in the missions, making new friends and all the stuff that comes with it, including lame or stupid people but those are everywhere anyways
Cheers