Yields like this are relativly easy too get. Just get your climate in order, use GOOD soil (plagron soil mixtures), use decent lighting (HPS) and a good nutrient mix specially developed for cannabis plants (I use canna or Plagron). o yeah and one more...

don't fuck around too much with all kinds of additives that should do wonders too your plants. In the end there are only a few additives that really garantee a bigger yeild. And those are:

Mycorhizza fungi spores (symbiot) i.e AN pirahna
Bacterial mix (symbiot and bac culture) i.e. AN Voodoo Juice
PK-13/14
Enzymes
CO²

What lots of people don't understand is that when you don't have a decent basic setup (climate, lights an stuff) you can use all the additives in the world but you'll never have a good yield.