What conclusion did you arrive at? If you truly want to lay out in the rain in a bivy then the eVent top and whatever waterproof bottom you like would seem like the right combination.
I have a full Pertex (don't recall the actual fabric spec at the moment) bivy that I've got some use on but I fail to be rained on in it. Hopefully I can rectify that this winter. Anyway, it's heavy for a bivy but probably in the ballpark of what you're asking about. Something like this or an eVent bivy. You should still expect condensation inside, especially when the humidity is high, such as in a rain storm.
I'd concur with others like Tony who suggested pairing a lighter breathable bivy with a tarp though. That setup provides 3 ways to camp... bivy only, tarp only, or bivy+tarp and will be a lot more comfortable to ride out the storm in than a bivy only - but I'm guessing you know all of this and might have thought it would just be easier to use the a shelter with full protection in liu of tarp+bivy (which arguably could be easier, more comfortable, and possibly lighter).
Bivvies are sort of a conundrum. Personally I've found they work best in situations where I don't really need them to begin with; that's sort of a bitter irony. I do like them for bug protection, to pair with a floorless shelter in the snow, and in situations where I'd like a little extra protection, say from rain splatter or nighttime dew. My latest foray into bivy trials has been an M90 and Cuben (bottom) bivy from Borah Gear. The M90 seems like a pretty nice material for durability and minimal protection (no comment on longevity yet). I realize this is not what you asked about but just thought I'd leave it as a parting thought.