For now just save your money. The couple bulbs you are already using, as long as they are 6500k color temp, will be fine. As the plant gets bigger, more of them would be helpful.
The color temperature and wattage are unrelated. Wattage just determines how much electricity the bulb uses. The higher the wattage, the more light the bulb puts out, thus the more the plant will like it.
When you switch to flower, go with the BIGGEST WATTAGE 2700k bulbs you can find/afford. Some people use the 6500k bulbs for flowering...how benificial that might be for the plant, I don't know.
Hopefully that helps....