Excellent Jack. I took you post as describing the differential sink rate (as a result of density) related to mono, flouro or thread. The most apparent place I think that would be obvious is small dries, and that's what I tried to highlight in my post. In my experience, density differences between common leader and tippet materials don't affect catch rates. What seems to affect catch rates, to a much larger extent, is the ability to get a drag free float. In both dry fly fishing and nymphing, it seems that the people that can cast and mend to achieve a drag free drift or float will have much higher catch rates, using the same flies and fishing the same water, as those that can't. I carry both mono and flouro, and haven't been able to discern a difference in hooking rates. To me, the flouro does seem to hold up better to abrasion relater to stream abrasion and especially fish teeth after multiple fish on the same tippet.