I have used a single piece of flourocarbon for a leader. Compared to standard mono of equal diameter they seem to sink a bit faster. I would expect that given my understanding the flouro is more dense. I haven't tried a furled leader made from flouro, but would expect it one to sink slower than a single strand flouro just as with regular mono. Fishing in the turbulent water of the surfzone with a sinking line and weighted fly I don't find much benefit for flourocarbon over standard mono. Seems to make more of a difference with floating line/nymphing setups.