I got the chance to test out the Bivy in some really wet and rainy conditions this weekend and wanted to post an update.
Weather during the two nights I tested the bivy was very, very wet. I was camping above the ocean, in a very thick fog/mist, with drizzling rain all night and a period of heavy sustained rain for about 3 hours during the middle of the night. Since I was using a non-DWR nylon down bag, it was very important that the bivy keep my bag dry. It certainly lived up to the test and exceeded my expectations.
The bivy is for purposes of tarp-camping, much more water resistant than a DWR nylon bivy. Despite the fact that condensation and rain was pouring down even inside my shelter, the bivy kept my bag dry. (I was somewhat disappointed with my shelter in these conditions as when the rain was pouring down it felt like it was raining inside my shelter. Everything in my shelter, other than the clothes in a silnylon dry sack and my bag in the bivy was absolutely soaked. I imagine this was due to the condensation/mist being pummelled off the shelter by the hard rain.)
Furthermore, a puddle formed under me during the night and water did not soak up into the bag from below. Since I was using a 48" length pad and no ground cloth, the only thing separating the bag from the wet ground was the bivy, and I did not notice any water coming through.
The one thing necessary to make this bivy truly stormproof in these conditions is a zipper or other means of closing it up. Water was splashing in from above and around my shelter into the opening of the bivy, and I eventually just scooted my bag and pad down to hunker down under the bottom half of the bivy to keep dry.
The next day I dried out the bivy and my sil-nylon shelter in the sun and noticed that the bivy dried more quicky than the shelter. It did accumulate lots of dirt and debris which is not easy to get off, but again, I didn't use a ground cloth and that could be avoided easily by doing so. (I wanted to test the waterproofness of the bivy underneath to see if water would come in from below--hence no ground cloth.)
Here are some pictures of the softened bivy:
Thom has offered to make me a bivy sized smaller for the use with a smaller neo-air pad. I'm hoping he can make use of some mesh to create a bug proof bivy as well as a zipper to fully enclose the bivy for stormy conditions.