The max flow rate listed on the UV light refers to the max flow rate the housing can handle without leaking... it has nothing to do with effectiveness...
I've used UVs for years while raising Blue Dempseys which are known to be prone to parasitic infections when young...
I use the following formula to eradicate parasites as well as all other UV benefits:
1W of UV strength per 10 Gal of water in the system (tank + sump)
10 gph per 1W of UV strength
In other words a 30W UV light at an actual flow rate of 300 gph on a 300 gal tank...
While it is true that a larger diameter housing will allow a longer dwell time at a given flow rate... The farther from the bulb the UV waves get the weaker they get... therefore a fat housing is not as good of an idea as it sounds when only considering flow rates.
Another point to consider... any particles that pass through the UV's housing will cast little shadows blocking UV rays. While a few particles here and there may not be a very big deal, it should be known that the cleaner the water is that passes through the UV housing the more effective your UV light will be.
I have used a lot of UV lights... and I have talked to a lot of people considering using them... I have never talked to anyone who 'properly' set up a UV light (I consider 'proper' following my suggested formula) that was not impressed with the result... but I have talked to quite a few people who cut corners and were not impressed. So my suggestion is, if you're going to do it, do it right...
As for brands... The Turbo Twist lights are nice, I have a couple of them. The best UV light I have ever seen is made by Danner. I also have a couple of them...