This is because no matter what, there's a hard cap at 150k coins and 40 coups per attempt. You could do 10 mill damage and someone else could do 2 mill, but as long as you are both over the cap (I don't actually know where it is, only that I hit it) you would both get the rewards I stated above. If you're all hitting the cap without the buffs, then I hate to tell you that you wasted your coups on the buffs /: