Definitely a money saver, and relatively quick as you found. Surprised there wasn't a drain plug gasket; sometimes they're black from the factory, which disguises them, but if you've had oil changes done since then, maybe they forgot to put it on at your last change.
Regarding overfilling; Subarus are more tolerant of a slight overfill vs under filling to a point, that point being when the lobes on the crank start turning the oil into a latte. Check your oil after the car has been sitting overnight. Use the distance between L and F to gauge how much extra oil you have (distance between L and F = ~1L = ~1 qt). If it's above the "hot" notch above F, then you're approaching too much.