Toppings effect will make the one main stem start to grow into 2 that will shoot off of the main it will slow the plant by a week or so while it heals from the HST that was done to it, but thats it.
I agree with dubba, topping set my plants back about a week. I like supercropping better than topping. It's similar to topping but different. I tried it on a couple of my soil plants, and you can REALLY see the difference. Compared to topping, it doesn't take as long to heal. Check out this thread by trillions of atoms. It's very useful, and I am glad he posted it. Check it out, see if you like it: http://www.marijuanapassion.com/forum/showthread.php?t=23201&highlight=supercropping+101