From what I understand, the high flow rate for algae/bacteria is based on 2 factors; 1. They can be killed with a short exposure to the UV (shorter than parasites), and 2. Since they reproduce very quickly, a faster flow allows the UV to treat more water and keep up or outpace their regrowth in the system.
I believe you are correct about this-- but I would expect that there is a diminishing return situation here: for it to be perfect you might want 1000 gph, but you would probably get 80% of the way there with 400 gph. For ich etc, 80% of the way there is completely unacceptable in my book, but for inhibiting micro algae and bacteria, my bet is that the observable difference in the tank that results from the remaining 20% is negligible.
