There's a thread that deals with this in the advanced cultivation section. If memory serves: 10w UVB bulbs do NOT have enough UVB to make any difference at all. Matter of fact, the guy doing the tests found only one or two bulbs that WORK for the job, and they are really expensive. a 5-watt is laughable really.
Lab tests did show an increase in THC when sufficient UVB was used. But the people to answer this question thoroughly hang out in the advanced section so maybe try post the same question there, I can't for the life of me remember WHO it was that did the actual tests and posted it, but yeah you will get some nicely educated answers from those guys. You'll probably also get some arguments but hey par for the course LOL.
Far as I remember the smallest useful one was 35 watts...