Sorry, Charlie.

There are a couple of things you can do to keep from getting burned. One is to order swatches of material from someone so you have a known fabric for comparison. owfinc.com has a good selection. The second thing is, you can do a suck test in the store. See if you can suck air through the fabric. You can feel air come through uncoated fabric - whereas it might hold a small puddle of water for a while due not to DWR, but to the lubricants used during weaving.

I've found silnylon at Wallyworld only once (many claims to the contrary may be like your experience) and it was a rather heavy, strangely woven stuff. Not what I am used to. It might work for a pack. Yeah, that's an idea.