I think you're correct. I had a mycobacterial infection in a tank that caused me to get a UV unit, and I thought that most pathogens required somewhere on the order of 20 or 25 watts for efficacy. Even at that, you had to adjust the flow rate so the water was exposed to the UV light for enough time to scramble the DNA. Another thing, I don't believe UV kills bacteria; I think it just messes up the DNA enough to prevent replication. (Microbiologists, jump in and correct me if I'm wrong here!)
Jim