Davy is the SLF expert and I'm sure that he will personally reply to your question. My philosophy is that as long as it's the color I want and it rolls on the thread the way I want it to, I use it whether I am tying nymphs or dries. I've often thought that the shiny characteristic of synthetics inhanced the effectiveness of a fly whether it floated or was sunken. Additionally, synthetics seem to be more resistant to discoloration from water that contains lots of algae.