Charlie, There are limits to everything , as you push the pH upwards they are less and less a forgiving fish as far water quality. They evolved as a soft water acidic pH fish.. the sweet spot is in the 5-7 range. Can they survive higher? Yes most of the time they can, though honestly no one ever studied the way pH affects Longevity in Discus. If a hobbyist must keep Discus in the 8 range, then water quality, health and diet are key to keeping them happy and healthy.
al