Eric, excellent reply. Really appreciated reading it.
1) I use a Wild Things Epic treated nylon windshirt - works well, but as you say, not quite as breatheable as SOME windshirts. I seems to me to be more breatheable than my GoLite Wisp (both '04 & newer Ether-fabric '05) and is probably more breatheable than the newer Marmot Ion windshirts. I like the WT Epic windshirt for use under a poncho in long-term &/or heavy rain as it keeps my forearms dry for hours, whereas a more commonplace DWR windshirt, while generally more breatheable, wets through faster (10-20 minutes typically), depending upon the rain. The Epic windshirt also, in some cases, allows me to refrain from donning full rain gear, poncho, or cape when the rain is lighter and long lasting.
2) DrJ once enlightened me on why heavier fabrics, like eVENT & GTX are not optimal choices for a windshirt. His info was the increased "pumping" effect/losses that a heavier fabric can produce due to one's body moving about (bending, moving, etc.), resulting in the very thing that an UL fabric windshirt is trying to avoid, viz. convective heat losses due to cold air exchange.
Again, very articulate post you authored. Thanks.