Paul, love your shelter system. "But it could be lighter!"
Yeah, yeah, I hear (say) that all the time.
But the fact remains you have a pretty killer setup for foul conditions at a very good weight.
I have an eVENT Unishelter and I shall now come out of the closet. "I use this 2 pound bivy sack". Whoo. Feeling better already.
In the winter, all cozied up in that sucker, with Armageddon swirling around you in a vicious storm, something just "feels good" about being inside it, all toasted up in about 8 inches of down, reading a nice book with a Photon light.
The performance, simplicity and weather protection of this system, especially above treeline, is well worth the weight, much of the time.
OK. I've shared too many of my feelings. I need to recover this post now.
You claim 5 deg increase in temp rating of your system using the eVENT Uni. I hear those numbers thrown around a lot, and have a hard time substantiating them. Passively, zero wind, you have two issues fighting each other: the accumulation of moisture in the system, which robs body heat to maintain the temperature, or change the phase state of that moisture, and the added insulation provided by the fabric and the air pocket between the fabric and your sleeping bag.
Where a bivy shines: wind.
No question, a bivy can have a huge impact on how warm you sleep in the wind. And I'm not talking about 30 mph gales - even a 3 mph wind in cold temperatures can cripple a light sleeping bag. The bivy has a big impact on sleeping warm in windy conditions because that insulating air layer that it traps - remains relatively still - and thus, warm.
Whew. That was close. Gotta watch those warm and fuzzies. Back to cutting the tags off my Uni...