I'm certainly no expert on fabrics. Read a bit. Asked the same questions you asked. Have used Epic in the rain a few times.
Shelters: Black Diamond Lightsabre bivy shelter.
I've spoken with a fellow at BD & he told me that the first thing he did when he bought a Lightsabre was to set it up in the yard, climb in, zip 'er up, & have his wife spray the bivy at close range for 20min. This was a forceful spray to be sure - He said the water made quite a racket striking the taut bivy fabric - louder than any rain would produce striking the fabric. He said no leaking occurred. I'm only guessing, but wouldn't rain strike the bivy with a lot less force than from the garden hose? Wouldn't this equate to less chance of water penetration?
I've used a BD Lightsabre bivy, made of Epic, in the rain. It was light/mod rain consisting of relatively small drops for most of the 'wee' hours of the night. The bivy did NOT wet through. I had it partially unzipped near the center top (there's a slight overhang/rain-flap so wind-driven rain won't enter the bivy), so exhaled water vapor could escape. Did not have much condensation in the morning - very little actually, even though it was raining outside. I could imagine that sometimes, i.e. under certain conditions there would be more condensation.
The only reason why I don't use this bivy more is that we can, at times, get all-day/all-night heavy rain & even for a couple of hours torrential downpours - you could stay drier in the shower at home!!! At this point, with everything, I've read, I don't trust Epic to withstand wetting through under such extreme conditions. At this point, I would only use it with a synth. sleeping bag if I knew the weather forecast said that it was going to rain.
I think some people have trouble distinguishing b/t condensation & leakage & blame the fabric for leaking when it was actually condensation build-up the produced the moisture in the tent/bivy. Also, is the Epic bivy seam-sealed? Is the leaking coming through the fabric or the unsealed seams?
Clothing: WildThings Epic Windshirt
As cp. to the GoLite Wisp & Montane Aero Windshirts (which are NOT Epic & only have a DWR finish) that I've used, the WT Epic Windshirt is certainly more waterproof. I wear it alone in long term lt. rain & mod. short duration rain. I keep it on underneath my poncho when the rain is either hvy. or long term mod. rain. I've worn it for a couple hours or so in the rain (not heavy rain) & it has NOT wet through.
By comparison, the other two windshirts, will stay dry, in my experience, for typically 10-20min of lt rain. I haven't had the Aero in the rain long enough for it to wet through. I have had the Wisp wet through in places (especially near the cuffs/wrists) when exposed to 10min of mod rain. Yes, it dried fast since it soaked up so little water. It's actually interesting to watch it dry. The parts that are wetted actually dry faster, than the drops "standing tall" on the fabric & have not wetted the fabric due to the DWR finish. This difference in drying/evaporation times is, of course, a surface area to volume issue & readily explainable.
Below Freezing Temps:
I read somewhere on this website that BPL discovered a problem with Epic that they were researching further. When the temp outside is below freezing, even with the temp inside the shelter is above freezing, the following can occur. [hope i recall the explanation correctly.]
1) water vapor condenses on the inside of the Epic fabric (this is necessary for the Epic fabric to wick the liquid water through the fabric & is not a bad thing)
2) the water, as it is transported through the thin Epic fabric, reaches a point where it freezes as it nears the outside of the fabric.
3) this causes the "pores", so to speak, or actually spaces b/t the threads(?), to become closed, or in other words, for the fabric to loose its' ability to breathe.
4) condensation really builds up in the tent when this situation occurs.
I think that's about all I know & can tell you.