I think this idea would be about the least comfortable thing i could ever dream up.
A tarp lying on top of you just pegged out, you'll feel every drop of rain, your escaping vapor will be trapped right on you (worse than the worst condensation you've ever seen in a tent or tarp)Head room, nope, read a book at night, nope, breath comfortably, nope, exit fast for late night pees, um nope.
This concept is far more extreme than i would ever be willing to go, the 8oz cuben tarp is money. Sew a few cinch straps to your quilt to bring it into lock down and leave the bivy at home, or go with a top bag next time and forget about crawlies. If the bivy's job is too keep bugs out and splash away i'd replace it with a larger tarp to fix the splash issue and some wedding vial mesh to keep bugs off your face. I just don't like or see the need for bivys. You build a great tarp with good head and elbow room then crawl into a claustrophobic waterproof envelope and seal yourself in. It's not for me.
But if you really want a bivy make it with silnylon floor to forget about ground sheets, momentum top for ultimate breathability and put it under a tarp for real weather protection.
I know that tarp-less camping can be done, but we could also attach arms to our butts. Just because we can doesn't mean we should.
Obviously i have an opinion on this, please don't take it personally. Everyone must find the kit that works best for them, I know that, it's just that my way IS THE BEST!
Sorry couldn't resist