From the picture, that looks exactly like my Sea to Summit Poncho Tarp. If it is, it's made out of ripstop nylon, is listed by the manufacturer as weighing 13 oz, and is a good shelter but not breathable. I've switched over to using it as my primary shelter until I can buy a new one, but I've only slept under it once. In that instance, I suffered major condensation and got rather wet (mostly due to warm, still weather on an open mountaintop). Overall, its good for your purpose--it's only needed for scenarios where it is likely to rain and you absolutely cannot get to a shelter.
I'd take the poncho and leave the bivy. Golite is restocking their poncho tarps on October 18, so you might want to spend the $60 to get a 7 oz breathable alternative.
You might want to consider a Gossamer Gear Polycryo groundcloth for $10, or buying the polycryo directly from Home Depot (find instructions for what to ask for somewhere on the site, I can't remember exactly).
With the sleeping bag adjustment, that gets you below 7 lbs. That's pretty solid--I doubt you'll get much lower without investing a lot of money, which may not be worth it.