As long as you keep the fish in a clean peaceful tank of good size and feed the fish a good diet and keep up on regular water changes, the goldfish should be fine. Goldfish don't actually have that fantastic eyesight, they rely on their other senses a great deal to find their way around the tank and locate food, its why blind goldfish tend to have no trouble swimming around and stuff, so even if the goldfishes eyes are covered a bit by its wen it shouldn't have any problems with this.
Large wen growth will always be a problem with oranges as a variety of goldfish in general, i never heard of anyone trimming back a goldfishes wen before, but to be honest i think such an operation would cause far more harm to the fish in comparison to any benefets it may have- oranda's have a lot of blood vessels in the wen which could cause a lot of bleeding and stress to the fish if they were damaged (not only that, but the wounds could be fatally infected), and it would also cause the fish a lot of pain (and yes, it has been scientifically proven that fish like goldfish do feel pain).