I gave up on the idea of a bivy for two reasons:
1. I have a plain ole' vanilla 1.3 sil MYOG tarp that measures 7.5'x8.5'and weighs 10.5oz counting the Ti stakes & triptease guys. While the top of a bivy would require both less & lighter fabric (perhaps M90), bottoms are typically heavier with @ least 1.3.
However, with a tarp, I can get away with a 1.5oz piece of 84"x40" window shrink wrap, (because it isn't sewn on) for a grand total of 12oz. Since the tarp is slightly oversized, I don't need anything else to get through a downpour.
2. I originally started with 6 yds, so the tarp began as a 9'x11.5' after the center seam was sewn. But since I measured & configured the tarp for myself (while inside), I was careful to make sure it was both high enough (38" at the front, 18" at the rear), and wide enough so that I would be clear from rubbing/wiping any surface area(s) condensation.
With bivy sacks, it just seems like there are always condensation issues, and lots of debates/explanation as to the reasons, including whether it is operator or equipment failure, etc.
I figure, who needs it? While it would be nice to throw a bivy down and crawl inside, the extra 5-10 minutes of erecting the tarp help keeps me dry, and hence warm.