Do not LST a plant that is already in flower, that will freak it out. Yes, if you do it within the first few weeks it won't freak out AS much but it will still tweak.

If you're already flowering, try again in your next grow at the right time, read stinky's post.

As far as yield increases, why don't you experiment and find out. There are so many different ways that one can increase the yield of their plant.