Lighting doesn't speed up flowering time. Thats mostly, a genetic trait.
In any case, too little lighiting may extend the flowering period if you use too little light and have too many plants. What happens is the top half of the plant flower finishes on time, but the lower half of the plants needs more time since they saw a crap load less light. Usually they're a week or two behind, but nothing major. You won't have this issue if you use a lot of lighting and don't over pack too many plants in one area.
For six full sized plants I'd use a 1000w. You need at least 200w of light per full sized plant. Any less and you may have the problem I wrote about above or you may get crapper yields. Choosing the right amount of plants is just as important as the amount of light you use.
