Would love to find some BD nanosphere. There is propore well under 2 oz, but it won't last very long.
The lightest ever available WPB I've found is Epic Malibu, which varies in weight, but is about 1.85 oz per sq. yd. on the average. No doubt you know there is great controversy over whether it fails in heavy rain, and by how much, especially if contaminated by soil and/or oily substances. Inasmuch as WPB clothing appears to require more water resistant material than tent/tarp walls, a bivy would probably be more like clothing, and Epic treatments have not worked well at all for clothing worn in the rain.
I have some 2.3 oz emerald colored nylon with an off white WPB coating from one of the UK online outlets (maybe Point North - they called it 'pertex,' but of course it is not). Roger tested it, and it is quite WP, though. PM me if you would like a sample. Everything else WPB seems to be 2.5 oz or more.
Where on earth did you find 1.6 oz eVent material, in any form. Please tell me.
I wouldn't kill for it, but almost.
AS FOR THE BOTTOM: The silnylon currently sold by Thru-Hiker is quite waterproof, based on tests by Richard in consultation with Roger and reported on this site. Even my little remaining best quality silnylon manufactured years ago is not as water resistant. The expansion (sag) with temp drops and cold rain or snow is still there, but not so much. Don't see how that would be a problem with a bivy bottom, though. Although it might be too slippery for some - not me who constantly rotates from one side to the other - but don't use a bivy anway, so not an issue.
Even after reading tons of posts, am still trying to decide between the 1.36 oz T-H sil and the thicker mylar 1.26 oz cuben from Zpacks for a tent floor. Certainly the sil will compress and pack better, be quieter, and acquire less sticky gunk. Theoretically, the elasticity of the sil would resist punctures and abrasion better. But that cuben is tough stuff. What to do. The answer would probably affect choice of a bivy bottom, also.