A plant will not finish faster if you increase the dark period. Increasing the light hours definitely will not fatten the buds up at all. If anything you will reveg and stress your plant. There has been speculation, however, that the potency will go up if you increase the dark period.
Before I get some angsty posts from "growers" I'd like to point out the science behind it. If you don't know what meristems are, look them up, they're important. In short, the SAM (shoot apical meristem) is your source of all the above ground "organs" in your mj plant. They are the plant stem cells, and what is grown during your vegetative period. Once the hours get shorter, and more Fr light wavelength is present (red spectrum lighting, found in your hps. That's why people say to flower with a hps) your plant goes from growing SAM to growing inflorescence meristems which then turn into floral meristems. That's why there is a one week period of stretch from veg to flower. Floral meristems are what produces your flowers (buds), and need around 12 hours of darkness to continue growing.
So what happens if you put the light on for 16 hours? Well, the floral meristems slow down in growth. This actually makes your buds "less fat." Funny enough, if you keep doing this for an extended period of time then the floral meristems will revert back to SAM... oh no! What does that mean? Your future growth will be vegetative growth, and you will have effectively revegged your plant.
Obviously only raising the amount of light you give your plants for a short period of time won't reveg the plant, but it won't do anything beneficial either, and has the potential to be harmful if anything. Also, I bet your electric bill is high enough, why pay more for it?
Now, onto less light.
The way THC is put on your buds (or at least is thought to) is a protection mechanism. The plant is protecting the buds from the sunlight. I have read, there really aren't any scientific books about mj so I don't know accurate this is, that THC is applied during the dark period. It's a sort of sunscreen basically that gets put on each night. Do not get confused though and think that your buds are what's telling your plant to put on the THC. Your leaves are the plants indicator of how much light it is getting, and how much THC ought to be put on. So if you chop all your leaves during flowering you are effectively getting way less out of your plant. Back to the sunscreen, the theory is that during the end of flowering your plants want to put on a lot more sunscreen as there is more area to cover and more need for it. So that's where you get people saying lower your light schedule and leave the last three days dark. I don't know how accurate any of this is though, however, I have done it myself with fine results.
