Chances won't be increased. Current chances and items drop are same as previous event in which we have statistic how much people get from it. I already see people with stage 10 and higher bows. High boxes doesn't contains usless items so we think chances are correct.
66% chance is already 2/3.
Also in case with increase chances, we will have to remove almost everything from reward list or add junk into high tear boxes or just make same boxes for every level.