I think you are probably on the right track Don, assuming that the assumptions don't make ass's of us all that is
I remember having a similar discussion last year (or maybe the year prior) when talking about winter feeding. What it basically came down to regarding algae grazing was that they derive roughly the same amount of energy from it that it takes to move it though their gut. The colder the water the less they eat AND the less it is digested, even though it spends up to 4 days in the gut. The digestive enzymes are simply running at 1 or 2% efficiency on a food low in nutrient value in the first place.
I know in our own experience and from comparing notes with others that the algae feces come out looking like tiny little round bales of green hay. Still very green and fibrous, which is a clear indication that while it may fill the gut, no real digestion for extracting nutrition takes place.
(yeah, I know.... Ace Ventura, Turd Inspector
)