UV is perhaps the oldest debated item online aside from salt brands to use, there simply is no concession from the crowd. That being said, I love uv and will always have one at least on backup if i owned a large tank, ive saved countless tanks from various water borne invaders by using UV but it was grossly oversized, it was pond UV's hooked to an aquarium.
When you match UV to the ratings the manufacturer states, and flow, you may or may not get your intended outcome. that sounds like a decent match above.
It doesnt prevent ich if thats a goal however....its great for preventing cyano and dinoflagellates and some algae and thats for sure.
The only way to ensure you hit a home run with uv is to grossly oversize it, we used to run 20 watts on a 15 gallon nano reef with dino invasions from our nano-reef.com peroxide thread and that did kill back his dinos. it was not a permanent setup, it was temporary and the uv was longer than the entire tank. it was oversized
there is no harm from over doing them, but underdoing them, or matching the requirements its rated for, sure could wind up being a waste of money.
whether or not UV kills tank plankton doesnt matter, its why we have to feed tanks. if the plankton in the tank mattered, we wouldnt need to feed.