FC has a refractive index of 1.42 with water being 1.33, nylon is around 1.56-1.58 depending upon the type of nylon.
Yes, and the refractive index of air is roughly 1.0, far enough different from either mono or fluoro that it ain't gonna make a difference. As far as the meniscus, yes, I know fluoro is hydrophobic, but to be honest in my home aquarium tests both are equally visible and distort the surface when on it, though fluoro has a tendency to sink more.
And then there's the surface film, which is a different substance than both the water and the air (hence why bugs can go to the surface easily but have trouble breaking through), and has a different refractive index yet (higher, though I've seen various estimates for how much higher and it varies depending on a lot of things). Not to mention if there's a small film on the water of a foreign substance, like virtually EVERY waterway has all the time. Most plant oils, for instance, have refractive indexes around 1.45-1.55. Things like salinity, temperature, dissolved oxygen, mineral content, etc. all affect the refractive index of water as well, though the effect is generally small.
You add it all up, and its a difficult question scientifically, to the point where you throw up your arms and trust your eyes in home tests, and I can say I see no difference in visibility ON the surface, if anything the mono is less visible cause it sits higher. Under the surface, its a different ball game, though even then the effect is overstated.
But in the end, it comes down to this:
Most fish are drag shy, NOT line shy. If you want to prove this to yourself, glue some various tippets onto Japanese beetles or hoppers and throw them into the drift. They all get taken just fine even with the pickiest trout, and even if you have 0x attached. But when you put it on your line, suddenly they're shy and finer tippets somehow help. Drag shy, not tippet shy, the finer tippets are causing less micro-drag. So:
The time to use a fluoro carbon tippet with dry flies is;
For selective trout, on smooth flat water, in bright light.
I disagree wholeheartedly. Thats the worst time to use it. Here you want the highest floating, limpest material you can find.
Where fluoro has a marked advantage is in abrasion resistance, which is useful for toothy creatures, so I can understand the love from the saltwater guys as well as the musky/pike crowd and even large trout. It also doesn't break down over time, again if long-term strength is of concern. The lack of stretch can be an advantage for some things as well, but a disadvantage in others.