John did you ever try the dual bivy concept? I live in the SW and like the Exped concept. In the SW most rainstorms don't last a long time like in other parts of the US. When it does rain, I pull over the bivy and everything is inside. Sometimes I keep hiking or sit on a log or rock until it passes.
If I have to make camp with rain, I can set up camp without getting wet. I install a couple of stakes at the arm holes from the inside, leave the head hole open for venting and use my trekking pole to prop the open end open. It works great. Ventilation is important and can be accomplished by leaving the open end, arm holes and head hole open downword.
It also works well for hammock camping because it is open on both sides. You can slide the entire bivy around the hammock providing protection from rain above and wind from below. When dry,I just push it on to one end and it is ready to pull over if it rains or gets colder.
It is great to change in or take care of pit stops while raining or snowing.
In my case it replaces my tent,tarp or normal bivy, and my goretex rain gear (about 3 to 5 lbs.). It works for short rain spells and certainly keeps you warmer without all the weight. It also provides emergency shelter for two or even three.