Driver and Fairway wood weights

I recently purchased a 913D2 and a 913 Fd. I like them and am hitting them well but they seem quite a but lighter than my previous clubs. I noticed that you sell a weight kit. What would adding a heaver weight do to the ball flight and club performance? Are the kits readily available or do I have to go to a certified fitter? I would like a little heftier feel and a little more head weight. Are individual weights available or do I need to buy the whole kit? Thanks for your assistance

That is an interesting question, I would not think a significant difference since it is in the center of the sole of the club...