Warren writes:
the leader/tippet diffracts the
light in very clear waders and leaves a "shadow" when on the surface
that causes very difficult, educated trout to flee.

we deal with that phenomenon here in the East a lot. Note Roger's comment, in
another thread, about starting to fish dries during the hatches to the good
fish on a downstream drift. Fly first, no leader to spot. Harvey did cite his
study as proving that one seldom needed less than 5X tippet for any trout fly
fishing. Even for midges and tricos he recommended 6X as a minor tactic. I
think he may have been correct, so long as the tippet is out of the trouts
view. Worth noting, the tests mentioned involved beetles, which get grabbed
quickly(and decided upon equally quickly) by most trout. A more deliberately
consumed food item(ex:abundant mayflies)
might produce different data.
Tom