I wont lie, mine don't get their water changed quite as much as my fish, but their tank still stays clean. I've also never dechlorinated the water and never had problems.
I also agree that a UGF is a very bad idea. I actually recently changed my turtle tank over to bare bottom to make it easier to remove all of the waste during water changes.
And as to the the salmonella thing...the 4" law from what I have heard has more to do with children handling them and being exposed to salmonella than the actual size of the turtle vs. it carrying the disease. Children are prone to put small things in their mouths, and a turtle, even from a very clean tank, is not something you want them to put there. Once over 4", turtles are much more difficult for kids to handle in that way, and the risk of transmitting salmonella decreases somewhat.