Keep in mind that topping is a stress training technique to help the plant grow out rather than straight up.. Or like a busy and not a Christmas tree. It's not really about yield.. Yes, you get more tops.. But the number of tops doesn't equal yield. Depending on the pheno of the plant, topping can increase, decrease or have no effect at all on yield. The purpose of topping is to break the apical dominance of the main cola. Doing this sends a signal to the plant to focus it's energy on growing side branches, rather than just the one main cola. All of the branches start to race to be the dominate cola, which is the one tallest branch or the closest to the light.