First of all...UGF's are actually good filters BUT are a huge pain to maintain properly...reverse UGF's are better because they push waste up instead of pulling it down (and therefore clogging up the filter).
Secondly...higher volume turnover is preferred because it basically allows waste products to be removed faster.
Lastly...the "inch of adult fish per gallon" is a guideline, and in reality the number of fish you can/should keep is based on a bunch of factors. Yes over filtration helps, but that alone is not going to allow you to have more fish. How well you clean your tank, how often, what fish you have, etc are all factors. This is why even experienced fish keepers like to err on the side of caution.