The thing about growing outside is that the sunlight is just as powerful at the lower parts if the plant as the top. The sunlight has travelled 93 million miles, so a few more feet won't make a difference. Also, around 90% of the sun's powerful light passes through the leaves, providing light to the dense center of the plants. Indoor lights, no matter how powerful, just don't match. That's why you top, trim, and super crop to get an even canopy, allowing as much surface area as possible, to take advantage of the limited light.
I don't know if topping outside gains anything. I would think if you're growing outside, as nature intended, plants should be left alone to grow naturally. When growing inside, trying to imitate nature as much as possible, plant training greatly improves yield.
