Sorry for not getting back sooner. I assisted at a fly casting clinic at the local college tonight and am just now getting home.
As far as fluorocarbon no longer being invisible once it has been furled:
Fluorocarbon is a chemical that has nearly the same light refraction rate as water.
Water refraction = 1.32
Fluorocarbon refraction = 1.42
Mono refraction = 1.53-1.62
When you make a furled leader you are twisting, very tight, 10 strands of fluorocarbon at the butt secion, 8 strands at the mid section and 6 strands at the tippet end. Once you have twisted this to shrink it about 10% of the original length, you then let the whole thing furl together. My opinion is that one strand of fluorocarbon has refraction properties very close to water, but, once that many strands are twisted and furled together, it will lose it’s refraction properties much like ducksterman stated. It is no longer a straight single strand, but, many strands twisted. Trust me, I could be wrong. All I know is that another gentleman that I know who makes furled leaders and is an engineer, told me that once twisted and furled, it will lose the refraction properties.
Fly Goddess
6/0 is probably the best if you are a dry fly fisherman and need the furled leader to float, but, the Coats & Clark 100% Nylon Quilting Thread will work the same as 6/0 but it will not absorb water nor retain water that might lead to mildew if put away wet. Another friend of mine, that is a guide and fly shop owner use to use the 6/0 furled leader all the time, but, since trying the .005 Coats & Clark furled leader, he has switched to using the Coats & Clark all the time. For my style of fishing I prefer the either fluorocarbon or mono furled leader because I am a nymph/wet fly/streamer fisherman and I want my flies to sink. The 100% Nylon has pretty close to the same density as water and will try to float on it’s own, mono will sink and fluorocarbon sinks faster than mono. I will make a furled leader out of 6/0 and break test it and let you know how that came out because I also want to know what 6/0 tests at. I will get back to you.
Old Bald Guy
Since you create your end loops with a double surgeons knot, I would say that your furled leaders would test a lot stronger because you are eliminating the weakest link. Just curious though, doesn’t the surgeons knot make for a bulky knot at each end? If my time will permit me, I may create a furled leader and use a Perfection loop in each end (only because I much prefer the Perfection knot to the Surgeons knot)and break test it for you and will let you know.
Ducksterman
That is hard to explain. They each broke at the loop on the tippet end of each furled leader. Only one broke on the leader side of the Shorb knot which allowed the leader to unfurl. All the others broke on the tippet loop side of the shorb knot and the leader stayed together.
One important point everyone needs to keep in mind is that we broke them with one steady hard straight line pull until something had to give. Once a furled leader is on your fly line on your fly rod, it would almost be impossible to break one unless you pointed the rod at the fish/snag and backed up with a steady straight pull until something gives. Your fly line stretch and rod flex will absorb a lot of the stress and keep even your tippet from breaking. Much the same with breaking fly line. If you tied the fly line around a tree and attempted to break it with pulling back on your rod, I think all you would accomplish is breaking your rod, but, it you pointed the rod at the tree and started backing up, you could break the fly line. A lot of people do not realize how powerful a fly rod flex is and how fast a fly rod can wear down a fighting fish.
Well, I hope I have been able to answer most of your questions to your satisfaction and remember I will be posting my break test results with furled leaders made from 6/0 tying thread, mono line and furled leaders with a perfection knot at each end for creating the end loops.
“Tune in later for the rest of the story” 
