I floated that section once years ago. The water quality is actually pretty good until you get below the Englewood WWTP outfall, then gets a little smelly, but like you said, still better than at Confluence and much much better than the SP below the Metro plant. I was actually surprised at how pleasant the float was, with lots of ducks geese, and a few herons, and thick vegetation hiding the urban setting for much of the reach.
If you have realistic expectations ("its not gonna be a pristine mtn stream") you may be pleasantly surprised.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse