I appreciate the thought, but 2.5 pounds is a little too far across the weight vs function line for me.
Of course I drool over the SUL bivies, but I haven't seen one yet that wasn't really a sleeping bag cover and meant for use with a small tarp or poncho.
I'm liking specs of the BD Twilight and Bill's description of the use of a bivy is really what it is about for me: a weatherproof haven that only needs a 2x6 flat spot. And i understand the compromises.
I have a solo tarp tent that is 11oz, a ponchmo that is 7oz, plus a bivy that needs a tarp and weighs 15oz.
The whole appeal is having this small light shelter to take on short trips that you can throw down next to the campfire like an anscestor did with a mastodon hide, roll up in it and go to sleep. You rise at dawn, make your breakfast at your elbow and break camp in 5 minutes. No poles, strings or stakes to fuss over and no worries about it hooking the wind and sailing off at 3 am.
So, a romantic notion of simple camping, but it still needs to work. A wet cold night isn't good! I have plenty of sticks and strings if I want more guaranteed comfort.
Thanks for the input!