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I would tend to agree on many setups, however if your running that high of test all the way down, you should never (rarely) have a tackle failure. I would say, in general, that a stiffer line (flasher to fly) will get the fly in a larger rotation. However, smaller diameter / more limp line will get that fly "twerking" a bit more with less rotation. Have to figure out what the fish want! Always easier said than done.

Having said all that - we run 40-50lb test to our flies more often than not.

I run 30 and 40 pound fluorocarbon on my flies. 30 on the small 1 1/2 inchers to 3-inchers I use early in the season and 40 on the big flies and spin -n glo laker rigs. The only reason I don't scale down to 20 pound line is because of the abrasion on the line caused by the fish teeth. Also, because when I pull a fish in the boat and out of the net, I grab the dodger and lift on the fish to try to keep the fish under control for a second or two to keep it from twisting/tangling in the net. If you have 20 pound, abraded leader and a pissed off fish, half tangled in the net, you just might break the leader. If I'm catching coho on 30 pound leader and see it's rubbed a bit, I'll usually send it back out. Same with 40 on big flies. I'll retie at the end of the trip, but I don't want to stop in the middle and start rethreading the flies, beads and making the loop knots.

As far as the different strengths of line giving more "flip" to the fly - I think that's really over rated or at least over thinking. If you want to think that, go ahead. If you are really convinced, then why not rig your flies with wire leader? I think adding or subtracting an inch or two to the leader probably changes the action of a fly more than switching from 20 to 40 pound leader. I do believe using fluoro rather than mono "occasionally" produces more strikes, but I've caught hundreds of fish on mono - and so have many if they use the flies right from the package. Only recently have some fly retailers switched to fluoro. If an occasional fish snubs mono but will bite fluoro, it's due mostly to line visibility. If that's the case, then remember, 30 pound is more visible than 20, 40 is easier for a fish to see than 30, 50 pound fluoro is probably more visible than 30 mono. Doesn't take long for things to start offsetting.

Sounds like cheep line or your not changing it often enough. In the spring I put on new line Mid season I put on new line. Always check the first 10 ft for nixs I only use 20# and I have caught 30# kings several times ( never horse a big fish )