Differences....
Hmmm,.... Guess I wasn't clear in trying to explain this. let's say that In considering the way it looks to a fish a visible, sinking poly leader would appear the same as a sink-tip or sinking line in the water. Therefore, IMHO, adding a lengthy (10' as you originally suggested) level tippet to a poly leader might be seen as the same as adding the same length of tippet to a sink-tip fly line. So, it seems to me that fish could be lined using either set-up.
Whether using a poly leader or flouro/mono leader the problem is the same, trying to avoid lining fish. In my experience (which doesn't include using poly leaders, BTW) the potential for lining fish at depth isn't a great concern. Admittedly, I do not fish Chironomids or very tiny flies as a rule (can't see 'em to tie 'em on ).
So,.... as I said before, to each his own.... Hope I didn't make this any more confusing than I have already....
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